=begin pod

=for comment
    This file is deliberately specified in Perl 6 Pod format
    Clearly a Perl 6 -> Perl 5 documentation translator is a high priority ;-)


=head1 TITLE

Synopsis 26 - Documentation


=head1 AUTHOR

Damian Conway <L<C<damian@conway.org>|mailto:damian@conway.org>>


=head1 VERSION

=table
    Maintainer:     Damian Conway
    Date:           9 Apr 2005
    Last Modified:  14 Feb 2007


=head1 Perldoc

Perldoc is an easy-to-use markup language with a simple, consistent
underlying document object model. Perldoc can be used for writing
language documentation, for documenting programs and modules, as well as
for other types of document composition.

Perldoc allows for multiple syntactic I<dialects>, all of which map onto
the same set of standard document objects. The standard dialect is named
"Pod".


=head1 The Pod Dialect

B<Pod> is an evolution of Perl 5's L<I<Plain Ol' Documentation>|doc:perlpod>
(POD) markup. Compared to Perl 5 POD, Perldoc's Pod dialect is much more
uniform, somewhat more compact, and considerably more expressive. The
Pod dialect also differs in that it is a purely descriptive mark-up
notation, with no presentational components.


=head2 General syntactic structure

Pod documents are specified using D<directives>, which are used to
declare configuration information and to delimit blocks of textual content.
Every directive starts with an equals sign (C<=>) in the first column.

The content of a document is specified within one or more D<blocks>. Every
Pod block may be declared in any of three equivalent forms:
L<I<delimited style>|#Delimited blocks>, L<I<paragraph style>|#Paragraph
blocks>, or L<I<abbreviated style>|#Abbreviated blocks>.

Anything in a document that is neither a Pod directive nor contained
within a Pod block is treated as "ambient" material. Typically this
would be the source code of the program that the Pod is documenting. Pod
parsers still parse this text into the internal representation of the
file (representing it as a C<Perldoc::Block::Ambient> block), but
renderers will usually ignore such blocks.

In Perl 5's POD format, once a POD directive is encountered, the parser
considers everything that follows to be POD, until an explicit C<=cut>
directive is encountered, at which point the parser flips between POD
and ambient text. The Perl 6 Pod format is different. A Pod parser
always reverts to "ambient" at the end of each Pod directive or block.
To cause the parser to remain in Pod mode, you must enclose the desired
Pod region in a C<pod> block:

=begin code :allow<B>
    B<=begin pod>

    =head1 A heading

    This is Pod too. Specifically, this is a simple C<para> block

        $this = pod('also');  # Specifically, a code block

    B<=end pod>
=end code

Alternatively you can indicate an entire file contains only Pod, by
giving it a C<.pod> suffix.


=head3 Delimited blocks

Delimited blocks are bounded by C<=begin> and C<=end> markers, both of
which are followed by a valid identifierN<A valid identifier is a
sequence of alphanumerics and/or underscores, beginning with an
alphabetic or underscore>, which is the D<typename> of the block. Typenames
that are entirely lowercase (for example: C<=begin head1>) or entirely
uppercase (for example: C<=begin SYNOPSIS>) are reserved.

After the typename, the rest of the C<=begin> marker line is treated as
configuration information for the block. This information is used in
different ways by different types of blocks, but is always specified using
Perl6-ish option pairs. That is, any of:

=for table :nested
  Value is...      Specify with...      Or with...      Or with...
  ===============  ==================   ==============  ======================
  Boolean (true)   C«:key»              C«:key(1)»      C«key => 1»
  Boolean (false)  C«:!key»             C«:key(0)»      C«key => 0»
  String           C«:key<str>»         C«:key('str')»  C«key => 'str'» 
  List             C«:key<1 2 3>»       C«:key[1,2,3]»  C«key => [1,2,3]» 
  Hash             C«:key{a=>1, b=>2}»                  C«key => {a=>1, b=>2}» 
  Code             C«:key{ sqrt($_) }»
   
All option keys and values must, of course, be constants since Perldoc
is a specification language, not a programming language.
See L<Synopsis 2|http://dev.perl.org/perl6/doc/design/syn/S02.html#Literals>
for details of the various Perl 6 pair notations.

The configuration section may be extended over subsequent lines by
starting those lines with an C<=> in the first column followed by a
whitespace character.

The lines following the opening delimiter and configuration are the
data or contents of the block, which continue until the block's C<=end>
marker line. For most block types, these contents may be indented if you
wish, without them being treated as L<code blocks|#Code blocks>. Unlike
Perl 5, indented text is only treated as code within C<=pod>,
L<C<=nested>|#Nesting blocks>, L<C<=item>|#Lists>, C<=code>, and
L<semantic|#Semantic blocks> blocks.

The general syntax is:

=begin code :allow< R >
     =begin R<BLOCK_TYPE>  R<OPTIONAL CONFIG INFO>
     =                  R<OPTIONAL EXTRA CONFIG INFO>
     R<BLOCK CONTENTS>
     =end R<BLOCK_TYPE>
=end code

For example:

     =begin table  :caption<Table of Contents>
         Constants           1
         Variables           10
         Subroutines         33
         Everything else     57
     =end table

     =begin Name  :required
     =            :width(50)
     The applicant's full name
     =end Name

     =begin Contact  :optional
     The applicant's contact details
     =end Contact

Note that no blank lines are required around the directives; blank
lines within the contents are always treated as part of the contents.
This is a universal feature of Pod.

Note also that in the following specifications, a "blank line" is a line
that is either empty or that contains only whitespace characters. That
is, a blank line matches the Perl 6 pattern: C</^^ \h* $$/>. Pod uses
blank lines as delimiters, rather than empty lines, the principle of
least surprise.


=head3 Paragraph blocks

Paragraph blocks are introduced by a C<=for> marker and terminated by
the next Pod directive or the first blank line (which is I<not>
considered to be part of the block's contents). The C<=for> marker is
followed by the name of the block and optional configuration
information. The general syntax is:

=begin code :allow< R >
     =for R<BLOCK_TYPE>  R<OPTIONAL CONFIG INFO>
     =                R<OPTIONAL EXTRA CONFIG INFO>
     R<BLOCK DATA>
=end code

For example:

     =for table  :caption<Table of Contents>
         Constants           1
         Variables           10
         Subroutines         33
         Everything else     57

     =for Name  :required
     =          :width(50)
     The applicant's full name

     =for Contact  :optional   
     The applicant's contact details


=head3 Abbreviated blocks

Abbreviated blocks are introduced by an C<'='> sign in the
first column, which is followed immediately by the typename of the
block. The rest of the line is treated as block data, rather than as
configuration. The content terminates at the next Pod directive or the
first blank line (which is not part of the block data). The general
syntax is:

=begin code :allow< R >
     =R<BLOCK_TYPE>  R<BLOCK DATA>
     R<MORE BLOCK DATA>

=end code

For example:

     =table
         Constants           1
         Variables           10
         Subroutines         33
         Everything else     57

     =Name     The applicant's full name
     =Contact  The applicant's contact details

Note that abbreviated blocks cannot specify configuration information. If
configuration is required, use a C<=for> or C<=begin>/C<=end> instead.


=head3 Block equivalence

The three block specifications (delimited, paragraph, and abbreviated)
are treated identically by the underlying documentation model, so you
can use whichever form is most convenient for a particular
documentation task. In the descriptions that follow, the abbreviated
form will generally be used, but should be read as standing for all
three forms equally.

For example, although L<#Headings> shows only:

     =head1 Top Level Heading

this automatically implies that you could also write that block as:

     =for head1
     Top Level Heading

or:

     =begin head1
     Top Level Heading
     =end head1


=head3 Standard configuration options

Pod predefines a small number of standard configuration options that can be
applied uniformly to built-in block types. These include:

=begin item :term<C<:nested>>

This option specifies that the block is to be nested within its current
context. For example, nesting might be applied to block quotes, to textual
examples, or to commentaries. In addition the L<C<=code>|#Code blocks>,
L<C<=item>|#Lists>, L<C<=input>|#I/O blocks>, and L<C<=output>|#I/O blocks>
blocks all have implicit nesting.

Nesting of blocks is usually rendered by adding extra indentation to the
block contents, but may also be indicated in others ways:
by boxing the contents, by changing the font or size of the nested text,
or even by folding the text (so long as a visible placeholder is provided).

Occasionally it is desirable to nest content by more than one level:

    =begin para :nested
    =begin para :nested
    =begin para :nested
    "We're going deep, deep, I<deep> undercover!"
    =end para
    =end para
    =end para

This can be simplified by giving the C<:nested> option a positive integer
value:

=for code :allow<B>
    =begin para B<:nested(3)>
    "We're going deep, deep, I<deep> undercover!"
    =end para

You can also give the option a value of zero, to defeat any implicit
nesting that might normally be applied to a paragraph. For example, to
specify a block of code that should appear I<without> its usual
nesting:

=for code :allow<B>
    =comment Don't nest this code block in the usual way...
    B<=begin code :nested(0)>

                 1         2         3         4         5         6
        123456789012345678901234567890123456789012345678901234567890
        |------|-----------------------|---------------------------|
          line        instruction                comments
         number           code

    =end code

Note that C<:!nested> could also be used for this purpose:

    =begin code :!nested

=end item

=begin item  :term<C<:numbered>>

This option specifies that the block is to be numbered. The most common
use of this option is to create L<numbered headings|#Numbered headings> and 
L<ordered lists|#Ordered lists>, but it can be applied to any block.

It is up to individual renderers to decide how to display any numbering
associated with other types of blocks.

=end item

=for item  :term<C<:term>>
This option specifies that a list item is the definition of a term.
See L<#Definition lists>.

=begin item  :term<C<:formatted>>

This option specifies that the contents of the block should be treated as if
they had one or more L<formatting codes|#Formatting codes> placed around them.

For example, instead of:

    =for comment
        The next para is both important and fundamental,
        so doubly emphasize it...

    =begin para
    B<I<
    Warning: Do not immerse in water. Do not expose to bright light.
    Do not feed after midnight.
    >>
    =end para

you can just write:

=for code :allow<B>
    =begin para B<:formatted<B I>>
    Warning: Do not immerse in water. Do not expose to bright light.
    Do not feed after midnight.
    =end para

The internal representations of these two versions are exactly the same,
except that the second one retains the C<:formatted> option information
as part of the resulting block object.

Like all formatting codes, codes applied via a C<:formatted> are
inherently cumulative. For example, if the block itself is already
inside a formatting code, that formatting code will still apply, in
addition to the extra "basis" and "important" formatting specified by
C<:formatted<B I>>.
=end item

=begin item :term<C<:like>>
This option specifies that a block or config has the same formatting
properties as the type named by its value. This is useful for creating
related L<configurations|#Block pre-configuration>. For example:

    =config head2  :like<head1> :formatted<I>

=end item

=for item  :term<C<:allow>>
This option expects a list of formatting codes that are to be recognized
within any C<V<>> codes that appear in (or are implicitly applied to)
the current block. The option is most often used on C<=code> blocks to
allow mark-up within those otherwise verbatim blocks, though it can be
used in I<any> block that contains verbatim text. See L<#Formatting
within code blocks>.


=head2 Blocks

Pod offers notations for specifying a range of standard block types...

=head3 Headings

Pod provides an unlimited number of levels of heading, specified by the
C<=head>R<N> block marker. For example:

    =head1 A Top Level Heading

    =head2 A Second Level Heading

    =head3 A third level heading

    =head86 A "Missed it by I<that> much!" heading

While Pod parsers are required to recognize and distinguish all levels
of heading, Pod renderers are only required to provide distinct
I<renderings> of the first four levels of heading (though they may, of
course, provide more than that). Headings at levels without distinct
renderings would typically be rendered like the lowest distinctly
rendered level.


=head4 Numbered headings

You can specify that a heading is numbered using the C<:numbered> option. For
example:

    =for head1 :numbered
    The Problem

    =for head1 :numbered
    The Solution

    =for head2 :numbered
    Analysis

    =for head3 
    Overview

    =for head3
    Details

    =for head2 :numbered
    Design

    =for head1 :numbered
    The Implementation

which would produce:

=begin nested :formatted<B>
1. The Problem

2. The Solution

=begin nested
2.1. Analysis

=begin nested
Overview

Details
=end nested

2.2: Design
=end nested

3. The Implementation
=end nested

It is usually better to preset a numbering scheme for each heading
level, in a series of L<configuration blocks|#Block pre-configuration>:

=for code :allow<B>
    B<=config head1 :numbered
    =config head2 :numbered
    =config head3 :!numbered>

    =head1 The Problem
    =head1 The Solution
    =head2   Analysis
    =head3     Overview
    =head3     Details
    =head2   Design
    =head1 The Implementation

Alternatively, as a short-hand, if the first whitespace-delimited word
in a heading consists of a single literal C<#> character, the C<#> is
removed and the heading is treated as if it had a C<:numbered> option:

    =head1 # The Problem
    =head1 # The Solution
    =head2   # Analysis
    =head3       Overview
    =head3       Details
    =head2   # Design
    =head1 # The Implementation

Note that, even though renderers are not required to distinctly render
more than the first four levels of heading, they I<are> required to
correctly honour arbitrarily nested numberings. That is:

    =head6 # The Rescue of the Kobayashi Maru

should produce something like:

=nested
B<2.3.8.6.1.9. The Rescue of the Kobayashi Maru>


=head3 Ordinary paragraph blocks

Ordinary paragraph blocks consist of text that is to be formatted into
a document at the current level of nesting, with whitespace
squeezed, lines filled, and any special L<inline mark-up|#Formatting codes>
applied.

Ordinary paragraphs consist of one or more consecutive lines of text,
each of which starts with a non-whitespace character at column 1. The
paragraph is terminated by the first blank line or block directive.
For example:

    =head1 This is a heading block

    This is an ordinary paragraph.
    Its text  will   be     squeezed     and
    short lines filled. It is terminated by
    the first blank line.

    This is another ordinary paragraph.
    Its     text    will  also be squeezed and
    short lines filled. It is terminated by
    the trailing directive on the next line.
    =head2 This is another heading block

Within a C<=pod>, C<=item>, C<=nested>, C<=END>, or
L<semantic|#Semantic blocks> block, ordinary paragraphs do not require
an explicit marker or delimiters, but there is also an explicit C<para>
marker (which may be used anywhere):

=for code :allow<B>
     B<=para>
     This is an ordinary paragraph.
     Its text  will   be     squeezed     and
     short lines filled.

and likewise the longer C<=for> and C<=begin>/C<=end> forms. For example:

=begin code :allow<B>
     B<=begin para>
         This is an ordinary paragraph.
         Its text  will   be     squeezed     and
         short lines filled.

         This is I<still> part of the same paragraph,
         which continues until an...
     B<=end para>
=end code

As the previous example implies, when any form of explicit C<para> block
is used, any whitespace at the start of each line is removed, so
the paragraph text no longer has to begin at column 1. In addition,
within a delimited C<=begin para>/C<=end para> block, any blank lines are
preserved.


=head3 Code blocks

Code blocks are used to specify pre-formatted text (typically source
code), which should be rendered without rejustification, without
whitespace-squeezing, and without recognizing any inline formatting
codes. Code blocks also have an implicit L<nesting|#Nesting blocks>
associated with them. Typically these blocks are used to show examples
of code, mark-up, or other textual specifications, and are rendered
using a fixed-width font.

A code block may be implicitly specified as one or more lines of text,
each of which starts with a whitespace character. The block is
terminated by a blank line. For example:

=begin code
    This ordinary paragraph introduces a code block:

        $this = 1 * code('block');
        $which.is_specified(:by<indenting>);
=end code

Implicit code blocks may only be used within C<=pod>, C<=item>,
C<=nested>, C<=END>, or L<semantic|#Semantic blocks> blocks.

There is also an explicit C<=code> block (which can be specified within
I<any> other block type, not just C<=pod>, C<=item>, etc.):

=begin code :allow<B>
     The C<loud_update()> subroutine adds feedback:

     B<=begin code>

     sub loud_update ($who, $status) {
         say "$who -> $status";

         silent_update($who, $status);
     }

     B<=end code>
=end code

As the previous example demonstrates, within an explicit C<=code> block
the code can start at the first column. Furthermore, lines that start
with whitespace characters have that whitespace preserved exactly (in
addition to the implicit nesting of the code). Explicit C<=code> blocks may
also contain empty lines.


=head4 Formatting within code blocks

Although C<=code> blocks automatically disregard all L<formatting
codes|#Formatting codes>, occasionally you may still need to specify
some formatting within a code block. For example, you may wish
to emphasize a particular keyword in an example (using a C<B<>> code). Or
you may want to indicate that part of the example is metasyntactic
(using the C<R<>> code). Or you might need to insert a non-ASCII
character (using the C<E<>> code).

You can specify a list of formatting codes that should still be
recognized within a code block using the C<:allow> option. The value of
the C<:allow> option must be a list of the (single-letter) names of one
or more formatting codes. Those codes will then remain active inside the
code block. For example:

    =begin code :allow< B R >
    sub demo {
        B<say> 'Hello R<name>';
    }
    =end code

would be rendered:

=begin code :allow< B R >
sub demo {
    B<say> 'Hello R<name>';
}
=end code

Although code blocks are verbatim by default, it can still occasionally
be useful to explicitly C<:allow> the verbatim formatting code (C<V<>>). That's
because, although the contents of an explicit C<=code> block are allowed to
start in column 1, they are not allowed to start with 
an equals sign in that first columnN<Because an C<=> in the first column is
I<always> the start of a Pod directive>. So, if an C<=> is needed in column 1,
it must be declared L<verbatim|#Verbatim text>:

=begin code :allow<B>
    =begin code :allow<V>

    B<V<=>> in the first column is always a Perldoc directive

    =end code
=end code


=head3 I/O blocks

Pod also provides blocks for specifying the input and output of
programs.

The C<=input> block is used to specify pre-formatted keyboard input,
which should be rendered without rejustification or squeezing of whitespace.

The C<=output> block is used to specify pre-formatted terminal or file
output which should also be rendered without rejustification or
whitespace-squeezing.

Note that, like C<=code> blocks, both C<=input> and C<=output> blocks have an
implicit level of nesting. They are also like C<=code> blocks in that they
are typically rendered in a fixed-width font, though ideally all three blocks
would be rendered in distinct font/weight combinations (for example: regular
serifed for code, bold sans-serif for input, and regular sans-serif for
output).

Unlike C<=code> blocks, both C<=input> and C<=output> blocks honour any
nested formatting codes. This is particularly useful since a sample of
input will often include prompts (which are, of course, output).
Likewise a sample of output may contain the occasional interactive
component. Pod provides L<special formatting codes|#Example specifiers>
(C<K<>> and C<T<>>) to indicate embedded input or output, so you can use
the block type that indicates the overall purpose of the sample (i.e. is
it demonstrating an input operation or an output sequence?) and then use
the "contrasting" formatting code within the block.

For example, to include a small amount of input in a sample of output:

=begin code :allow<B>
    =begin output
        Name:    Baracus, B.A.
        Rank:    Sgt
        Serial:  1PTDF007

        Do you want additional personnel details? B<K<y>>

        Height:  180cm/5'11"
        Weight:  104kg/230lb
        Age:     49

        Print? B<K<n>>
    =end output
=end code


=head3 Lists

Lists in Pod are specified as a series of contiguous C<=item> blocks. No
special "container" directives or other delimiters are required to
enclose the entire list. For example:

     The seven suspects are:

     =item  Happy
     =item  Dopey
     =item  Sleepy
     =item  Bashful
     =item  Sneezy
     =item  Grumpy
     =item  Keyser Soze

List items have one implicit level of nesting:

=begin nested
The seven suspects are:

=item  Happy
=item  Dopey
=item  Sleepy
=item  Bashful
=item  Sneezy
=item  Grumpy
=item  Keyser Soze
=end nested

Lists may be multi-level, with items at each level specified using the
C<=item1>, C<=item2>, C<=item3>, etc. blocks. Note that C<=item> is just
an abbreviation for C<=item1>. For example:

     =item1  Animal
     =item2     Vertebrate
     =item2     Invertebrate

     =item1  Phase
     =item2     Solid
     =item2     Liquid
     =item2     Gas
     =item2     Chocolate

which would be rendered something like:

=for para :nested
E<bull> Animal
=for para :nested(2)
E<ndash> Vertebrate
=for para :nested(2)
E<ndash> Invertebrate

=for para :nested
E<bull> Phase
=for para :nested(2)
E<ndash> Solid
=for para :nested(2)
E<ndash> Liquid
=for para :nested(2)
E<ndash> Gas
=for para :nested(2)
E<ndash> Chocolate

Perldoc parsers must issue a warning if a "level-R<N+1>" C<=item> block
(e.g. an C<=item2>, C<=item3>, etc.) appears anywhere except where there
is a preceding "level-R<N>" C<=item> in the same surrounding block. That
is, an C<=item3> should only be specified if an C<=item2> appears
somewhere before it, and that C<=item2> should itself only appear if
there is a preceding C<=item1>.

Note that item blocks within the same list are not physically nested.
That is, lower-level items should I<not> be specified inside
higher-level items:

    =comment WRONG...
    =begin item1          --------------
    The choices are:                    | 
    =item2 Liberty        ==< Level 2   |==<  Level 1
    =item2 Death          ==< Level 2   |
    =item2 Beer           ==< Level 2   |
    =end item1            --------------

    =comment CORRECT...
    =begin item1          ---------------
    The choices are:                     |==< Level 1
    =end item1            ---------------
    =item2 Liberty        ==================< Level 2
    =item2 Death          ==================< Level 2
    =item2 Beer           ==================< Level 2


=head4 Ordered lists

An item is part of an ordered list if the item has a C<:numbered>
configuration option:

     =for item1 :numbered
     Visito

     =for item2 :numbered
     Veni

     =for item2 :numbered
     Vidi

     =for item2 :numbered
     Vici

This would produce something like:

=begin nested
1. Visito

=begin nested
1.1. Veni

1.2. Vidi

1.3. Vici
=end nested
=end nested

although the numbering scheme is entirely at the discretion of the
renderer, so it might equally well be rendered:

=begin nested
1. Visito

=begin nested
1a. Veni

1b. Vidi

1c. Vici
=end nested
=end nested

or even:

=begin nested
A: Visito

=begin nested
E<nbsp;nbsp>(i) Veni

E<nbsp>(ii) Vidi

(iii) Vici
=end nested
=end nested

Alternatively, if the first word of the item consists of a single C<#>
character, the item is treated as having a C<:numbered> option:

     =item1  # Visito
     =item2     # Veni
     =item2     # Vidi
     =item2     # Vici

To specify an I<unnumbered> list item that starts with a literal C<#>, either
make it verbatim:

=for code :allow<B>
    =item B<V<#>> introduces a comment

or explicitly mark the item itself as being unnumbered:

=for code :allow<B>
    =for item B<:!numbered>
    # introduces a comment

The numbering of successive C<=item1> list items increments
automatically, but is reset to 1 whenever any other kind of non-ambient
Perldoc block appears between two C<=item1> blocks. For example:

    The options are:

    =item1 # Liberty
    =item1 # Death
    =item1 # Beer

    The tools are:

    =item1 # Revolution
    =item1 # Deep-fried peanut butter sandwich
    =item1 # Keg

would produce:

=begin nested
The options are:
=begin nested
=para 1. Liberty
=para 2. Death
=para 3. Beer
=end nested

The tools are:

=begin nested
=para 1. Revolution
=para 2. Deep-fried peanut butter sandwich
=para 3. Keg
=end nested
=end nested

The numbering of nested items (C<=item2>, C<=item3>, etc.) only resets
(to 1) when the higher-level item's numbering either resets or increments.

To prevent a numbered C<=item1> from resetting after a non-item block,
you can specify the C<:continued> option:

=begin code :allow<B>
     =for item1
     # Retreat to remote Himalayan monastery

     =for item1
     # Learn the hidden mysteries of space and time

     I<????>

     =for item1 B<:continued>
     # Prophet!
=end code

which produces:

=begin nested
=para 1. Retreat to remote Himalayan monastery
=para 2. Learn the hidden mysteries of space and time
=para I<????>
=para 3. Prophet!
=end nested


=head4 Definition lists

To create term/definition lists, specify the term as a configuration value
of the item, and the definition as the item's contents:

=begin code :allow<B>
    =for item  B<:term<MAD>>
    Affected with a high degree of intellectual independence.

    =for item  B<:term<MEEKNESS>>
    Uncommon patience in planning a revenge that is worth while.

    =for item  B<:term<MORAL>>
    Conforming to a local and mutable standard of right.
    Having the quality of general expediency.
=end code

An item that's specified as a term can still be numbered:

=begin code :allow<B>
    =for item B<:numbered> :term<SELFISH>
    Devoid of consideration for the selfishness of others. 

    =for item B<:numbered> :term<SUCCESS> 
    The one unpardonable sin against one's fellows.
=end code


=head4 Unordered lists

List items that do not specify either the C<:numbered> or C<:term> options are
unordered. Typically, such lists are rendered with bullets. For example:

    =item1 Reading
    =item2 Writing
    =item3 'Rithmetic

might be rendered:

=for para :nested(1)
E<bull;nbsp;nbsp>Reading
=for para :nested(2)
E<mdash;nbsp;nbsp>Writing
=for para :nested(3)
E<curren;nbsp;nbsp>'Rithmetic

As with numbering styles, the bulleting strategy used for different levels
within a nested list is entirely up to the renderer.


=head4 Multi-paragraph list items

Use the delimited form of the C<=item> block to specify items that
contain multiple paragraphs. For example:

     Let's consider two common proverbs:

     =begin item :numbered
     I<The rain in Spain falls mainly on the plain.>

     This is a common myth and an unconscionable slur on the Spanish
     people, the majority of whom are extremely attractive.
     =end item

     =begin item :numbered
     I<The early bird gets the worm.>

     In deciding whether to become an early riser, it is worth
     considering whether you would actually enjoy annelids
     for breakfast.
     =end item

     As you can see, folk wisdom is often of dubious value.

which produces:

=begin nested
=config item :numbered
Let's consider two common proverbs:

=begin item
I<The rain in Spain falls mainly on the plain.>

This is a common myth and an unconscionable slur on the Spanish
people, the majority of whom are extremely attractive.
=end item

=begin item
I<The early bird gets the worm.>

In deciding whether to become an early riser, it is worth
considering whether you would actually enjoy annelids
for breakfast.
=end item

As you can see, folk wisdom is often of dubious value.
=end nested


=head3 Nesting blocks

Any block can be nested by specifying an C<:nested> option on it:

=for code :allow<B>
    =begin para B<:nested>
        We are all of us in the gutter,E<NL>
        but some of us are looking at the stars!
    =end para

However, qualifying each nested paragraph individually quickly becomes
tedious if there are many in a sequence, or if multiple levels of
nesting are required:

=for code :allow<B>
    =begin para B<:nested>
        We are all of us in the gutter,E<NL>
        but some of us are looking at the stars!
    =end para
    =begin para B<:nested(2)>
            -- Oscar Wilde
    =end para

So Pod provides a C<=nested> block that marks all its contents as being
nested:

=for code :allow<B>
    B<=begin nested>
    We are all of us in the gutter,E<NL>
    but some of us are looking at the stars!
    B<=begin nested>
    -- Oscar Wilde
    B<=end nested>
    B<=end nested>

Nesting blocks can contain any other kind of block, including implicit
paragraph and code blocks.


=head3 Tables

Simple tables can be specified in Perldoc using a C<=table> block.
The table may be given an associated description or title using the
C<:caption> option.

Columns are separated by whitespace, vertical lines (C<|>), or border
intersections (C<+>). Rows can be specified in one of two ways: either
one row per line, with no separators; or multiple lines per row with
explicit horizontal separators (whitespace, intersections (C<+>), or
horizontal lines: C<->, C<=>, C<_>) between I<every> row. Either style
can also have an header row at the top (which must be separated from the
first content row by at least one non-whitespace horizontal separator
character).

Each individual table cell is separately formatted, as if it were a
nested C<=para>.

This means you can create tables compactly, line-by-line:

    =table
        The Shoveller   Eddie Stevens     King Arthur's singing shovel   
        Blue Raja       Geoffrey Smith    Master of cutlery              
        Mr Furious      Roy Orson         Ticking time bomb of fury      
        The Bowler      Carol Pinnsler    Haunted bowling ball           

or line-by-line with multi-line headers:

    =table
        Superhero     | Secret          | 
                      | Identity        | Superpower 
        ==============|=================|================================
        The Shoveller | Eddie Stevens   | King Arthur's singing shovel   
        Blue Raja     | Geoffrey Smith  | Master of cutlery              
        Mr Furious    | Roy Orson       | Ticking time bomb of fury      
        The Bowler    | Carol Pinnsler  | Haunted bowling ball           

or with multi-line headers I<and> multi-line data:

    =begin table :caption('The Other Guys')

                        Secret                                         
        Superhero       Identity          Superpower                     
        =============   ===============   ===================
        The Shoveller   Eddie Stevens     King Arthur's
                                          singing shovel   

        Blue Raja       Geoffrey Smith    Master of cutlery              

        Mr Furious      Roy Orson         Ticking time bomb
                                          of fury      

        The Bowler      Carol Pinnsler    Haunted bowling ball           

    =end table


=head3 Named blocks

Blocks whose names are not recognized as Pod built-ins are assumed to be
destined for specialized renderers or parser plug-ins. For example:

    =begin Xhtml
    <object type="video/quicktime" data="onion.mov">
    =end Xhtml

or:

    =Image http://www.perlfoundation.org/images/perl_logo_32x104.png

Named blocks are converted by the Perldoc parser to block objects;
specifically, to objects of a subclass of the standard
C<Perldoc::Block::Named> class.

For example, the blocks of the previous example would be converted to
objects of the classes C<Perldoc::Block::Named::Xhtml> and
C<Perldoc::Block::Named::Image> respectively. Both of those classes
would be automatically created as subclasses of the
C<Perldoc::Block::Named> class (unless they were already defined via a
prior L<C<=use directive>|#Modules>).

The resulting object's C<.typename> method retrieves the short name of
the block type: C<'Xhtml'>, C<'Image'>, etc. The object's C<.config>
method retreives the list of configuration options (if any). The
object's C<.contents> method retrieves a list of the block's
verbatim contents.

Named blocks for which no explicit class has been defined or loaded are
usually not rendered by the standard renderers.

Note that all block names consisting entirely of lower-case or entirely of
upper-case letters are reserved. See L<#Semantic blocks>.


=head3 Comments

Comments are Pod blocks that are never rendered by any renderer. They
are, of course, still included in any internal Perldoc representation,
and are accessible via the Perldoc API.

Comments are useful for meta-documentation (documenting the documentation):

    =comment Add more here about the algorithm

and for temporarily removing parts of a document:

=begin code :allow<B>
    =item # Retreat to remote Himalayan monastery

    =item # Learn the hidden mysteries of space and time

    =item # Achieve enlightenment

    B<=begin comment>
    =item # Prophet!
    B<=end comment>
=end code

Note that, since the Perl interpreter never executes embedded Perldoc
blocks, C<comment> blocks can also be used as (nestable!) block comments
in Perl 6:

    =begin comment
    for my $file (@files) {
        system("rm -rf $file");
    }
    =end comment



=head3 The C<=END> block

The C<=END> block is special in that all three of its forms
(L<delimited|#Delimited blocks>, L<paragraph|#Paragraph blocks>, and
L<abbreviated|#Abbreviated blocks>) are terminated only by the end of the
current file. That is, neither C<=END> nor C<=for END> are terminated by the
next blank line, and C<=end END> has no effect within a C<=begin END> block.
A warning is issued if an explicit C<=end END> appears within a document.

An C<=END> block indicates the end-point of any ambient material within
the document. This means that the parser will treat all the remaining
text in the file as Perldoc, even if it is not inside an explicit block. In
other words, apart from its special end-of-file termination behaviour,
an C<=END> block is in all other respects identical to a C<=pod> block.


=head3 Data blocks

Named Perldoc blocks whose typename is C<DATA> are the Perl 6 equivalent of
the Perl 5 C<__DATA__> section. The difference is that C<=DATA> blocks are
just regular Pod blocks and may appear anywhere within a source file, and as
many times as required.
L<Synopsis 2|doc:http://dev.perl.org/perl6/doc/design/syn/S02.html#Literals>
describes the new Perl 6 interface for inline data.


=head3 Semantic blocks

All other uppercase block typenames are reserved for specifying standard
documentation, publishing, or source components. In particular, all
the standard components found in Perl and manpage documentation have
reserved uppercase typenames.

Standard semantic blocks include:

    =NAME
    =VERSION
    =SYNOPSIS
    =DESCRIPTION
    =USAGE
    =INTERFACE 
    =METHOD
    =SUBROUTINE
    =OPTION
    =DIAGNOSTIC
    =ERROR
    =WARNING
    =DEPENDENCY
    =BUG
    =SEEALSO
    =ACKNOWLEDGEMENT
    =AUTHOR
    =COPYRIGHT
    =DISCLAIMER 
    =LICENCE
    =LICENSE
    =TITLE
    =SECTION
    =CHAPTER
    =APPENDIX
    =TOC
    =INDEX
    =FOREWORD
    =SUMMARY
    =DEFAULT
    =PURPOSE

The plural forms of each of these keywords are also reserved, and are
aliases for the singular forms.

Most of these blocks would typically be used in their full delimited forms:

=begin code
    =begin SYNOPSIS
        use Perldoc::Parser

        my Perldoc::Parser $parser .= new();

        my $tree = $parser.parse($fh);
    =end SYNOPSIS
=end code

Semantic blocks can be considered to be variants of the C<=head1> block
in most respects (and most renderers will treat them as such). The main
difference is that, in a C<=head1> block, the heading is the contents of
the block; whereas, in a semantic block, the heading is derived from the
typename of the block itself and the block contents are instead treated as
the C<=para> or C<=code> block(s) belonging to the heading.

The use of these special blocks is not required; you can still just write:

=begin code
    =head1 SYNOPSIS
    =begin code
        use Perldoc::Parser
        
        my Perldoc::Parser $parser .= new();
        
        my $tree = $parser.parse($fh);
    =end code
=end code

However, using the keywords adds semantic information to the
documentation, which may assist various renderers, summarizers, coverage
tools, document refactorers, and other utilities. This is because a
semantic block I<encloses> the text it controls (unlike a C<=head1>,
which merely precedes its corresponding text), so using semantic blocks
produces a more explicitly structured document.

Note that there is no requirement that semantic blocks be rendered in
a particular way (or at all). Specifically, it is not necessary to
preserve the capitalization of the keyword. For example, the
C<=SYNOPSIS> block of the preceding example might be rendered like so:

=begin nested
B<3.E<nbsp;nbsp>I<Synopsis>>

=begin code
    use Perl6::Perldoc::Parser;
        
    my $rep = Perl6::Perldoc::Parser.parse($fh, :all_pod);
=end code
=end nested


=head2 Formatting codes

Formatting codes provide a way to add inline mark-up to a piece of text
within the contents of (most types of) block. Formatting codes are
themselves a type of block, and most of them may nest sequences of any
other type of block (most often, other formatting codes). In particular,
you can nest comment blocks in the middle of a formatting code:

=for code :allow<B V>
    V<B><I shall say this loudly
    B<=begin comment
    and repeatedly
    =end comment>
    and with emphasis.>

All Pod formatting codes consist of a single capital letter followed
immediately by a set of angle brackets. The brackets contain the text or
data to which the formatting code applies. You can use a set of single
angles (C«<...>»), a set of double angles (C<«...»>), or multiple
single-angles (C«<<<...>>>»).

Within angle delimiters, you cannot use sequences of the same angle
characters that are longer than the delimiters:

=begin code :allow<B>
    =comment
        These are errors...

    C< $fooB«<<»barB«>>» >
    The Perl 5 heredoc syntax was: C< B«<<»END_MARKER >
=end code

You I<can> use sequences of angles that are the same length as
the delimiters, but they must be balanced. For example:

    C<  $foo<bar>   >
    C<< $foo<<bar>> >>

If you need an unbalanced angle, either use different delimiters:

=for code :allow<B>
    CB<«>$foo < $barB<»>
    The Perl 5 heredoc syntax was: CB<«> <<END_MARKER B<»>

or delimiters with more consecutive angles than your text contains:

=for code :allow<B>
    CB«<<»$foo < $barB«>>»
    The Perl 5 heredoc syntax was: CB«<<<» <<END_MARKER B«>>>»

A formatting code ends at the matching closing angle bracket(s), or at
the end of the enclosing block or formatting code in which the opening
angle bracket was specified, whichever comes first. Pod parsers are
required to issue a warning whenever a formatting code is terminated by
the end of an outer block rather than by its own delimiter (unless the
user explicitly disables the warning).


=head3 Significance indicators

Pod provides three formatting codes that flag their contents with
increasing levels of significance:

=item
The C<U<>> formatting code specifies that the contained text is
B<unusual> or distinctive; that it is of I<minor significance>. Typically
such content would be rendered in an underlined style.

=item 
The C<I<>> formatting code specifies that the contained text is
B<important>; that it is of I<major significance>. Such content would
typically be rendered in italics or in C< <em>...<em/> > tags

=item
The C<B<>> formatting code specifies that the contained text is the
B<basis> or focus of the surrounding text; that it is of I<fundamental
significance>. Such content would typically be rendered in a bold style or
in C< <strong>...</strong> > tags.


=head3 Definitions

The C<D<>> formatting code indicates that the contained text is a
B<definition>, introducing a term that the adjacent text
elucidates. For example:

=for code :allow<B>
    There ensued a terrible moment of B<D<coyotus interruptus>>: a brief
    suspension of the effects of gravity, accompanied by a sudden
    to-the-camera realisation of imminent downwards acceleration.

A definition may be given synonyms, which are specified after a vertical bar
and separated by semicolons:

=for code :allow<B>
    A B<D<Formatting code|formatting codes;formatters>> provides a way
    to add inline mark-up to a piece of text.

A definition would typically be rendered in italics or C< <dfn>...</dfn> >
tags and will often be used as a link target for subsequent instances of the
term (or any of its specified synonyms) within a hypertext.


=head3 Example specifiers

Perldoc provides formatting codes for specifying inline examples of input,
output, code, and metasyntax:

=begin item
The C<T<>> formatting code specifies that the contained text is
B<terminal output>; that is: something that a program might print out.
Such content would typically be rendered in a T<fixed-width font> or with
C< <code>...</code> > tags. The contents of a C<T<>> code are always
L<space-preserved | #Space-preserving text> (as if they had an implicit
C<S<...>> around them). The C<T<>> code is the inline equivalent of the
C<=output> block.
=end item

=begin item
The C<K<>> formatting code specifies that the contained text is
B<keyboard input>; that is: something that a user might type in. Such
content would typically be rendered in a K<fixed-width font> (preferably a
different font from that used for the C<T<>> formatting code) or with
C< <kbd>...</kbd> > tags. The contents of a C<K<>> code are always
L<space-preserved| #Space-preserving text>. The C<K<>> code is the
inline equivalent of the C<=input> block.
=end item

=begin item
The C<C<>> formatting code specifies that the contained text is B<code>;
that is, something that might appear in a program or specification. Such
content would typically be rendered in a C<fixed-width font> (preferably
a different font from that used for the C<T<>> or C<K<>> formatting
codes) or with C< <samp>...</samp> > tags. The contents of a C<C<>> code
are L<space-preserved| #Space-preserving text> and L<verbatim| #Verbatim text>.
The C<C<>> code is the inline equivalent of the C<=code> block.

To include other formatting codes in a C<C<>> code, you can lexically
L<reconfigure|#Block pre-configuration> it:

=begin code :allow<B>
    =begin para
    B<=config C<> :allow<E I>>
    Perl 6 makes extensive use of the C<B<E<laquo>>> and C<B<E<raquo>>>
    characters, for example, in a hash look-up:
    C<%hashB<I<E<laquo>>>keyB<I<E<raquo>>>>
    =end para
=end code

To enable entities in I<every> C<C<...>> put a C<=config C<> :allow<E>>
at the top of the document
=end item

=begin item
The C<R<>> formatting code specifies that the contained text is a
B<replaceable item>, a placeholder, or a metasyntactic variable. It is
used to indicate a component of a syntax or specification that should
eventually be replaced by an actual value. For example:

=for code :allow<B>
    The basic C<ln> command is: C<ln> B<R<source_file> R<target_file>>

or:

=begin code :allow<B>
    Then enter your details at the prompt:

    =for input
        Name: B<R<your surname>>
          ID: B<R<your employee number>>
        Pass: B<R<your 36-letter password>>
=end code

Typically replaceables would be rendered in R<fixed-width italics> or with
C< <var>...</var> > tags. The font used should be the same as that used for
the C<C<>> code, unless the C<R<>> is inside a C<K<>> or C<T<>> code (or
the equivalent C<=input> or C<=output> blocks), in which case their
respective fonts should be used.
=end item


=head3 Verbatim text

The C<V<>> formatting code treats its entire contents as being B<verbatim>,
disregarding every apparent formatting code within it. For example:

    The B<V< V<> >> formatting code disarms other codes
    such as V< I<>, C<>, B<>, and M<> >.

Note, however that the C<V<>> code only changes the way its
contents are parsed, I<not> the way they are rendered. That is, the
contents are still wrapped and formatted like plain text, and the
effects of any formatting codes surrounding the C<V<>> code
are still applied to its contents. For example the previous example
is rendered:

=nested
The B<V< V<> >> formatting code disarms other codes
such as V< I<>, C<>, B<>, and M<> >.

You can prespecify formatting codes that remain active within
a C<V<>> code, using the L<C<:allow>|#Formatting within code blocks>
option.


=head3 Inline comments

The C<Z<>> formatting code indicates that its contents constitute a
B<zero-width comment>, which should not be rendered by any renderer.
For example:

=for code :allow<B>
    The "exeunt" command B<Z<Think about renaming this command?>> is used
    to quit all applications.

In Perl 5 POD, the C<Z<>> code was widely used to break up text that would
otherwise be considered mark-up:

=for code :allow<B>
    In Perl 5 POD, the ZB<Z<>><> code was widely used to break up text
    that would otherwise be considered mark-up.

That technique still works, but it's now easier to accomplish the same goal 
using a verbatim formatting code:

=for code :allow<B>
    In Perl 5 POD, the B«V<»Z<>B«>» code was widely used to break up text
    that would otherwise be considered mark-up.

Moreover, the C<C<>> code automatically treats its contents as being
verbatim, which often eliminates the need for the C<V<>> as well:

=for code :allow<B>
    In Perl 5 POD, the B«C<»Z<>B«>» code was widely used to break up text
    that would otherwise be considered mark-up.

The C<Z<>> formatting code is the inline equivalent of a
L<C<=comment> block|#Comments>.


=head3 Links

The C<L<>> code is used to specify all kinds of links, filenames, citations,
and cross-references (both internal and external).

A link specification consists of a I<scheme specifier> terminated by a
colon, followed by an I<external address> (in the scheme's preferred
syntax), followed by an I<internal address> (again, in the scheme's syntax).
All three components are optional, though at least one must be present in
any link specification.

Usually, in schemes where an internal address makes sense, it will be
separated from the preceding external address by a C<#>, unless the
particular addressing scheme requires some other syntax. When new
addressing schemes are created specifically for Perldoc it is strongly
recommended that C<#> be used to mark the start of internal addresses.

Standard schemes include:

=begin item  :term('C<http:> and C<https:>')
A standard web URL. For example:

=for code :allow<B>
    This module needs the LAME library
    (available from L<B<http://www.mp3dev.org/mp3/>>)

If the link does not start with C<//> it is treated as being relative to
the location of the current document:

=for code :allow<B>
    See also: L<B<http:tutorial/faq.html>> and
    L<B<http:../examples/index.html>>

=end item

=begin item :term<C<file:>>

A filename on the local system. For example:

=for code :allow<B>
    Next, edit the global config file (that is, either
    L<B<file:/usr/local/lib/.configrc>> or L<B<file:~/.configrc>>).

Filenames that don't begin with a C</> or a C<~> are relative to the current
document's location:

=for code :allow<B>
    Then, edit the local config file (that is, either
    L<B<file:.configrc>> or L<B<file:CONFIG/.configrc>>.

=end item

=begin item :term<C<mailto:>>

An email address. Typically, activating this type of link invokes a mailer.
For example:

=for code :allow<B>
    Please forward bug reports to L<B<mailto:devnull@rt.cpan.org>>

=end item

=begin item :term<C<man:>>

A link to the system manpages. For example:

=for code :allow<B>
    This module implements the standard
    Unix L<B<man:find(1)>> facilities.

=end item

=begin item :term<C<doc:>>

A link to some other documentation, typically a module or part of the core
documentation. For example:

=for code :allow<B>
    You may wish to use L<B<doc:Data::Dumper>> to
    view the results. See also: L<B<doc:perldata>>.

=end item

=begin item :term<C<defn:>>

A link to the L<definition|#Definitions> of the specified term within
the current document. For example:

=for code :allow<B>
    He was highly prone to B<D<lexiphania>>: an unfortunate proclivity
    for employing grandiloquisms (for example, words such as "proclivity",
    "grandiloquism" and indeed "lexiphania").
    
and later, to link back to the definition

=for code :allow<B>
   To treat his chronic L<B<defn:lexiphania>> the doctor prescribed an
   immediate glossoligation or, if that proved ineffective, a complete
   cephalectomy.
    
=end item

=begin item :term<C<isbn:> and C<issn:>>

The International Standard Book Number or International Standard
Serial Number for a publication. For example:

=for code :allow<B>
    The Perl Journal was a registered 
    serial publication (L<B<issn:1087-903X>>)

=end item

To refer to a specific section within a webpage, manpage, or Perldoc
document, add the name of that section after the main link, separated by
a C<#>. For example:

=for code :allow<B>
    Also see: L<man:bash(1)B<#Compound Commands>>,
    L<doc:perlsynB<#For Loops>>, and
    L<http://dev.perl.org/perl6/syn/S04.htmlB<#The_for_statement>>

To refer to a section of the current document, omit the external address:

=for code :allow<B>
    This mechanism is described under L<doc:B<#Special Features>> below.

The scheme name may also be omitted in that case:

=for code :allow<B>
    This mechanism is described under L<B<#Special Features>> below.

Normally a link is presented as some rendered version of the link
specification itself. However, you can specify an alternate
presentation by prefixing the link with the desired text and a
vertical bar. Whitespace is not significant on either side of the bar.
For example:

=begin code :allow<B>
    This module needs the L<B<LAME library|>http://www.mp3dev.org/mp3/>.

    You could also write the code
    L<B<in Latin |> doc:Lingua::Romana::Perligata>
=end code


=head3 Placement links

A second kind of linkE<mdash>the C<P<>> or B<placement link>E<mdash>works
in the opposite direction. Instead of directing focus
out to another document, it allows you to draw the contents of another
document into your own.

In other words, the C<P<>> formatting code takes a URI and (where possible)
places the contents of the corresponding document inline in place of the
code itself.

C<P<>> codes are handy for breaking out standard elements of
your documentation set into reusable components that can then be
incorporated directly into multiple documents. For example:

    =COPYRIGHT
    P<file:/shared/docs/std_copyright.pod>

    =DISCLAIMER
    P<http://www.MegaGigaTeraPetaCorp.com/std/disclaimer.txt>

might produce:

=begin nested
B<Copyright>

This document is copyright (c) MegaGigaTeraPetaCorp, 2006. All rights reserved.

B<Disclaimer>

ABSOLUTELY NO WARRANTY IS IMPLIED. NOT EVEN OF ANY KIND. WE HAVE SOLD
YOU THIS SOFTWARE WITH NO HINT OF A SUGGESTION THAT IT IS EITHER USEFUL
OR USABLE. AS FOR GUARANTEES OF CORRECTNESS...DON'T MAKE US LAUGH! AT
SOME TIME IN THE FUTURE WE MIGHT DEIGN TO SELL YOU UPGRADES THAT PURPORT
TO ADDRESS SOME OF THE APPLICATION'S MANY DEFICIENCIES, BUT NO PROMISES
THERE EITHER. WE HAVE MORE LAWYERS ON STAFF THAN YOU HAVE TOTAL
EMPLOYEES, SO DON'T EVEN *THINK* ABOUT SUING US. HAVE A NICE DAY.
=end nested

If a renderer cannot find or access the external data source for a
placement link, it must issue a warning and render the URI directly in
some form, possibly as an outwards link. For example:

=begin nested
B<Copyright>

See: L<std_copyright.pod|file:/shared/docs/std_copyright.pod>

B<Disclaimer>

See: L<http://www.MegaGigaTeraPetaCorp.com/std/disclaimer.txt>
=end nested

You can use any of the following URI forms (see L<#Links>) in a
placement link:

=item C<http:> and C<https:>
=item C<file:>
=item C<man:>
=item C<doc:>
=item C<toc:>

The C<toc:> form is a special pseudo-scheme that inserts a table of contents
in place of the C<P<>> code. After the colon, list the block types that you
wish to include in the table of contents. For example, to place a table of
contents listing only top- and second-level headings:

    P<toc: head1 head2>

To place a table of contents that lists the top four levels of headings, as
well as any tables:

    P<toc: head1 head2 head3 head4 table>

To place a table of diagrams (assuming a user-defined C<Diagram> block):

    P<toc: Diagram>

Note also that, for C<P<toc:...>>, all L<semantic blocks|#Semantic
blocks> are treated as equivalent to C<head1> headings, and the
C<=item1>/C<=item> equivalence is preserved.


=head3 Space-preserving text

Any text enclosed in an C<S<>> code is formatted normally, except that
every whitespace character in itE<mdash>including any newlineE<mdash>is preserved.
These characters are also treated as being non-breaking (except for the
newlines, of course). For example:

    The emergency signal is: S<
    dot dot dot   dash dash dash   dot dot dot>.

would be formatted like so:

=nested
The emergency signal is:E<NEL>
dotE<nbsp>dotE<nbsp>dotE<nbsp>E<nbsp>E<nbsp>dashE<nbsp>dashE<nbsp>dashE<nbsp>E<nbsp>E<nbsp>E<nbsp>dotE<nbsp>dotE<nbsp>dot.

rather than:

=nested
The emergency signal is: dot dot dot dash dash dash dot dot dot.


=head3 Ambient aliases

The C<A<>> formatting code specifies an B<alias to an ambient antecedent>.
This is like a L<placement link|#Placement links>, except
that the text that is inserted to replace the C<A<>> formatting code is
some portion of the L<ambient section(s)|#General syntactic structure>
of the current document, rather than the entire contents of some
external document.

Hence, the C<A<>> code makes it possible to incorporate pieces of
ambient text (typically source code) into Pod documentation.
Specifically, the C<A<>> code is replaced by searching backwards through
all preceding non-Pod parts of the document, to locate the nearest prior
substring that matches the contents of the C<A<>> code.

The contents of an C<A<>> code can specify a back-reference of this type
in one of two ways:

=item as a I<prefix keyword>, or

=item as a I<delimited text range>.

By default, C<A<>> aliases are "keyword oriented". That is, the contents
of an C<A<>> block are treated as a keyword or prefix that introduces
the desired text. That text is located by searching backwards from the
location of the C<A<>> code, to find the nearest preceding instance of
the specified prefix in any previous ambient block. The text that is
then used to replace the C<A<>> is the first "symbol" following that
located prefix. In this context, a "symbol" is defined as a sequence of
non-whitespace characters terminated by a transition from an identifier
character to a non-identifier character.

For example, in the following:

=begin code
    class Pet {
    
        has $name;
    
    =DESCRIPTION
    The class A<class> provides a A<has> attribute.
=end code

the C<A<class>> formatting code would be replaced by "Pet", since that
is the sequence of non-whitespace characters that immediately follows
"class" in the preceding ambient source code. Likewise, the C<A<has>> 
formatting code would be replaced by "$name", because that is the
longest sequence of non-whitespace characters that follows a "has" and
terminates in an identifier-to-nonidentifier boundary.

=begin para
=config C<> :allow<R>
In other words, any formatting code of the form C<A<R<prefix>>>
is replaced by the substring of the nearest preceding
ambient block that matches the pattern:
=end para

=for code :allow<R>
    /  .*  R<prefix> \s*  <( \S*? \w )>  [\W | $] /

This default is designed to work well for the commonest kind of
back-reference in ambient text: a reference to a code construct that
was defined using a prefix keyword and whose name ends in an identifier.

The second and more general way of specifying an alias is to specify
both a prefix and a postfix delimiter for the replacement text. If the
contents of an C<A<>> formatting code include a range marker (C<..>),
the sequence before the C<..> is treated as the left delimiter of the
replacement text, and the sequence after the C<..> is the right
delimiter. In this case, there are no other constraints on the
replacement text. In particular, it may contain any number of non-
identifier or whitespace characters. For example:

    class Pet {

        method eat(Food $meal) {...}

    =for DESCRIPTION
    The A<method>() method has the following argument list: A<(..)>

This would be interpreted as:

    The eat() method has the following argument list: Food $meal

because the C<A<(..)>> specifies an alias to the closest preceding ambient
text that is left-delimited by '(' and right-delimited by ')'.

To specify an alias in which the sequence C<..> is itself
a left- or right-delimiter (rather than the separator between the two),
use a C<V<>> code:

    constant @range = 0..99;

    =para
    The maximum value is A<V<..>..;>

If the left delimiter is omitted, it defaults to "start of line".
If the right delimiter is omitted, it defaults to "end of line".
For example:

    =head3 Standard conversions

    constant %convert = (
        'cat' => 'dog',
    =item A«..=>» to A«=>..»

        'fox' => 'hen',
    =item A«..=>» to A«=>..»

        'man' => 'ape',
    =item A«..=>» to A«=>..»

    );


=head4 Explicit aliasing

The replacement strings for C<A<>> formatting codes are normally
specified implicitly, by the closest preceding ambient text that matches
the contents of the C<A<>> code.

However, it is possible to override this behaviour and create an
I<explicitly defined> alias, using the C<=alias> directive:

    class Agent {...}
    =alias component Agent

    class Transaction is Activity {

    =DESCRIPTION 
    The A<class> class represents a transaction activity between two
    A<component> objects.

In the preceding example, C<A<class>> is a normal "keyword" alias
(which would be replaced by the closest preceding prefixed match:
"Transaction"). However, C<A<component>> is a defined alias
(which would be replaced by the explicitly specified text: "Agent").

Each back-reference name defined by an <=alias> directive is lexically
scoped within the block structure of the surrounding Pod. To create
"global" aliases, define them at the start of the Pod document, at the
outermost block level.

Explicitly defined aliases always override normal prefix or delimited
aliases, and thereby allow you to refer to ambient constructs that would
otherwise be inaccessible to an implicit back-reference.

For example, within the C<DESCRIPTION> block of the previous example,
the Agent class couldn't be referred to as C<A<class>>, since the
intervening Transaction class "hides" it from the look-behind matching
of implicit back-reference mechanism. But the C<=alias> command allows
the name of the earlier class to be associated with a distinct symbolic
alias (i.e. "component"), which then allows it to be referred to
unambiguously, regardless of other intervening code.

Another way of thinking of this is that explicitly defined aliases
change the normal C<A<>> substitution behaviour from being determined
I<relatively> by the location of the C<A<>> code, to being determined
I<absolutely> by the alias name itself.

An C<=alias> directive expects two arguments:

=item The name of the new alias

=item The text with which that new alias is to be replaced

The alias name may be any sequence of non-whitespace characters. The
remainder of the line (ignoring the whitespace immediately after the
name) is treated as the replacement text. For example:

    =alias Smith  Jones
    =alias G&T    green tea
    =alias #*@%!  Gosh darn

    =para
    A<#*@%!> it, A<Smith>, you spilled my A<G&T>!

is equivalent to:

    =para
    Gosh darn it, Jones, you spilled my green tea!

Excessively long replacement strings can be specified across multiple
lines, using the standard Perldoc "extender" notation:

    =alias MIGSOTRSFTPOELATSNDUPSTOMIMD
    =  Member in Good Standing of the Royal Society for the Prevention
    =  of Excessively Long Acronyms that Serve No Discernably Useful
    =  Purpose save to Obfuscate Modern Instantly Messaged Discourse
 
To specify an alias name that includes significant whitespace, or a
replacement text with surrounding whitespace, use a C<V<>>
formatting code:

    =alias slow            V< >...slow
    =alias V<extra slow>   s-l-o-w
    =alias V<ultra slow>   V<  >s  l  o  wV<  >

    =para
    The service was not merely A<slow>, or even A<extra slow>.
    It was A<ultra slow>

Although only the C<V<>> code is significant within the name of an alias,
you can use I<any> formatting code(s) within the replacement text:

    =alias V<ultra slow>   S<  s  l  o  w  >
    =alias V<hyper slow>   B<...s...l...o...w...>

In particular, you can use an C<A<>> code in the replacement text of an
C<=alias>. This is useful to preserve the abstract relationship between
ambient code and Pod documentation. For example, in the earlier Agent
example, instead of:

    class Agent {...}
    =alias component Agent

the alias could have been defined:

    class Agent {...}
    =alias component A<class>

so that the class name did not have to be repeated as part of the alias.
This approach has the important benefit that the alias would not have
to be modified in any way if the name of the Agent class were
subsequently changed:

    class Operative {...}
    =alias component A<class>

Likewise, in the earlier range example, it would have been cleaner and
more maintainable to write:

    constant @range = 0..99;
    =alias max  A<V<..>..;>

    =para
    The maximum value is A<max>

Note that C<=alias> is a fundamental Perldoc directive, like C<=begin>
or C<=for>; it is I<not> an instance of an
L<abbreviated block|#Abbreviated blocks>. Hence there is no paragraph
or delimited form of the C<=alias> directive (just as there is no
paragraph or delimited form of C<=begin>).


=head3 Entities

To include named Unicode or XHTML entities, use the C<E<>> code.

If the contents of the C<E<>> are a number, that number is
treated as the decimal Unicode value for the desired codepoint.
For example:

    Perl 6 makes considerable use of E<171> and E<187>.

You can also use explicit binary, octal, decimal, or hexadecimal numbers
(using the Perl 6 notations for explicitly based numbers):

    Perl 6 makes considerable use of E<0b10101011> and E<0b10111011>.
    Perl 6 makes considerable use of E<0o253> and E<0o273>.
    Perl 6 makes considerable use of E<0d171> and E<0d187>.
    Perl 6 makes considerable use of E<0xAB> and E<0xBB>.

If the contents are not a number, they are interpreted as a
Unicode character name (which is always upper-case), or else as an XHTML
entity. For example:

    Perl 6 makes considerable use of E<LEFT DOUBLE ANGLE BRACKET>
    and E<RIGHT DOUBLE ANGLE BRACKET>.

or, equivalently:

    Perl 6 makes considerable use of E<laquo> and E<raquo>.

Multiple consecutive entities can be specified in a single C<E<>> code,
separated by semicolons:

    Perl 6 makes considerable use of E<laquo;hellip;raquo>.


=head3 Indexing terms

Anything enclosed in an C<X<>> code is an B<index entry>. The contents
of the code are both formatted into the document and used as the
(case-insensitive) index entry:

=for code :allow<B>
    An B<X<array>> is an ordered list of scalars indexed by number,
    starting with 0. A B<X<hash>> is an unordered collection of scalar
    values indexed by their associated string key.

You can specify an index entry in which the indexed text and the index
entry are different, by separating the two with a vertical bar:

=for code :allow<B>
    An B<X<array|arrays>> is an ordered list of scalars indexed by number,
    starting with 0. A B<X<hash|hashes>> is an unordered collection of
    scalar values indexed by their associated string key.

In the two-part form, the index entry comes after the bar and is
case-sensitive.

You can specify hierarchical index entries by separating indexing levels
with commas:

=for code :allow<B>
    An X<array|B<arrays, definition of>> is an ordered list of scalars
    indexed by number, starting with 0. A X<hash|B<hashes, definition of>>
    is an unordered collection of scalar values indexed by their
    associated string key.

You can specify two or more entries for a single indexed text, by separating
the entries with semicolons:

=for code :allow<B>
    A X<hash|B<hashes, definition of; associative arrays>>
    is an unordered collection of scalar values indexed by their
    associated string key.

The indexed text can be empty, creating a "zero-width" index entry:

=for code :allow<B>
    B<X<|puns, deliberate>>This is called the "Orcish Manoeuvre"
    because you "OR" the "cache".


=head3 Annotations

Anything enclosed in an C<N<>> code is an inline B<note>.
For example:

=for code :allow<B>
    Use a C<for> loop instead.B<N<The Perl 6 C<for> loop is far more
    powerful than its Perl 5 predecessor.>> Preferably with an explicit
    iterator variable.

Renderers may render such annotations in a variety of ways: as
footnotes, as endnotes, as sidebars, as pop-ups, as tooltips, as
expandable tags, etc. They are never, however, rendered as unmarked
inline text. So the previous example might be rendered as:

=nested
Use a C<for> loop instead.E<dagger> Preferably with an explicit iterator
variable.

and later:

=begin nested
B<Footnotes>

=para
E<dagger> The Perl 6 C<for> loop is far more powerful than its Perl 5
predecessor.
=end nested


=head3 User-defined formatting codes

L<Perldoc modules|#Modules> can define their own formatting codes,
using the C<M<>> code. An C<M<>> code must start with a
colon-terminated scheme specifier. The rest of the enclosed text is
treated as the (verbatim) contents of the formatting code. For example:

=begin code :allow<B>
    =use Perldoc::TT

    =head1 Overview of the B<M<TT: $CLASSNAME >> class
    (version B<M<TT: $VERSION>>)

    B<M<TT: get_description($CLASSNAME) >>
=end code

The C<M<>> formatting code is the inline equivalent of a
L<named block|#Named blocks>.

Internally an C<M<>> code is converted to an object derived from the
C<Perldoc::FormattingCode::Named> class. The name of the scheme becomes
the final component of the object's classname. For instance, the C<M<>>
code in the previous example would be converted to a
C<Perldoc::FormattingCode::Named::TT> object, whose C<.typename>
method retrieves the string C<"TT"> and whose C<.contents>
method retrieves a list of the formatting code's (verbatim,
unformatted) contents.

If the formatting code is unrecognized, the contents of the code (i.e.
everything after the first colon) would normally be rendered as
ordinary text.

=head2 Encoding

By default, Perldoc assumes that documents are Unicode, encoded in one
of the three common schemes (UTF-8, UTF-16, or UTF-32). The particular
scheme a document uses is autodiscovered by examination of the first few
bytes of the file (where possible). If the autodiscovery fails, UTF-8 is
assumed, and parsers may treat any non-UTF-8 bytes later in the
document as fatal errors.

At any point in a document, you can explicitly set or change the encoding
of its content using the C<=encoding> directive:

    =encoding ShiftJIS

    =encoding Macintosh

    =encoding KOI8-R

The specified encoding is used from the start of the I<next> line in
the document. If a second C<=encoding> directive is encountered, the
current encoding changes again after that line. Note, however, that
the second encoding directive must itself be encoded using the first
encoding scheme.

This requirement also applies to an C<=encoding> directive at the very
beginning of the file. That is, it must itself be encoded in
the default UTF-8, -16, or -32. However, as a special case, the
autodiscovery mechanism will (as far as possible) also attempt to
recognize "self-encoded" C<=encoding> directives that begin at the first
byte of the file. For example, at the start of a ShiftJIS-encoded file
you can specify C<=encoding ShiftJIS> in the ShiftJIS encoding.

An C<=encoding> directive affects any ambient code between the Perldoc
as well. That is, Perl 6 uses C<=encoding> directives to determine the
encoding of its source code as well as that of any documentation.

Note that, like C<=alias>, C<=encoding> is a fundamental Perldoc
directive, not an abbreviated block form. Hence there is no paragraph or
delimited form of the C<=encoding> directive.


=head2 Block pre-configuration

The C<=config> directive allows you to prespecify standard configuration
information that is applied to every block of a particular type.

For example, to specify particular formatting for different levels of
heading, you could preconfigure all the heading directives with
appropriate formatting schemes:

    =config head1              :formatted<B U>  :numbered
    =config head2 :like<head1> :formatted<I>
    =config head3              :formatted<U>
    =config head4 :like<head3> :formatted<I>

The general syntax for configuration directives is:

=for code  :allow< R >
    =config R<BLOCK_TYPE>  R<CONFIG OPTIONS>
    =                   R<OPTIONAL EXTRA CONFIG OPTIONS>

Like C<=alias> and C<=encoding>, a C<=config> is a directive, not a
block. Hence, there is no paragraph or delimited form of the C<=config>
directive. Each C<=config> specification is lexically scoped to the
surrounding block in which it is specified.

Note that, if a particular block later explicitly specifies a
configuration option with the same key, that option overrides the
pre-configured option. For example, given the heading configurations in the
previous example, to specify a I<non>-basic second-level heading:

    =for head2 :formatted<I U>
    Details

The C<:like> option causes the current formatting options for the
named block type to be (lexically) I<replaced> by the complete
formatting information of the block type specified as the C<:like>'s
value. That other block type must already have been preconfigured. Any
additional formatting specifications are subsequently added to that
config. For example:

=for code :allow<B>
    =comment  In the current scope make =head2 an "important" variant of =head1
    =config head2 B<:like<head1>> :formatted<I>

Incidentally, this also means you can arrange for an explicit C<:formatted>
option to I<augment> an existing C<=config>, rather than replacing
it. Like so:

=for code :allow<B>
    =comment  Mark this =head3 (but only this one) as being important
              (in addition to the normal formatting)...
    =head3 B<:like<head3>> :formatted<I>


=head3 Pre-configuring formatting codes

You can also lexically preconfigure a L<formatting code|#Formatting
codes>, by naming it with a pair of angles as a suffix. For example:

=for code :allow<B>
    =comment  Always allow E<> codes in any (implicit or explicit) V<> code...
    B<=config V<>  :allow<E>>

=for code :allow<B>
    =comment  All inline code to be marked as important...
    B<=config C<>  :formatted<I>>

Note that, even though the formatting code is named using single-angles,
the preconfiguration applies regardless of the actual delimiters used on
subsequent instances of the code.


=head2 Modules

Perldoc provides a mechanism by which you can extend the syntax,
semantics, or content of your documentation: the C<=use> directive.

Specifying a C<=use> causes a Perldoc processor to load the
corresponding Perldoc module at that point, or to throw an exception if
it cannot.

Such modules can specify additional content that should be included in
the document. Alternatively, they can register classes that handle new
types of block directives or formatting codes.

Note that a module loaded via a C<=use> statement can affect the
content or the interpretation of subsequent blocks, but I<not> the
initial parsing of those blocks. Any new block types must still
conform to the general syntax described in this document. Typically, a
module will change the way that renderers parse the contents of
specific blocks.

A C<=use> directive may be specified with either a module name or a URI:

=for code :allow< R >
    =use R<MODULE_NAME>  R<OPTIONAL CONFIG DATA>
    =                 R<OPTIONAL EXTRA CONFIG DATA>

=for code :allow< R >
    =use R<URI>

If a URI is given, the specified file is treated as a source of Pod
to be included in the document. Any Pod blocks are parsed out of the
contents of the C<=use>'d file, and added to the main file's Pod
representation at that point.

If a module name is specified, with a language prefix of C<pod:>, then
the corresponding C<.pod> file is searched for in the C<$PERL6DOC>
"documentation path". If none is found, the corresponding C<.pm> file is
then searched for in the library path (C<$PERL6LIB>). If either file is
found, the Pod is parsed out of it and the resulting block objects
inserted into the main file's representation.

If a module name is specified with any prefix except C<pod:>, or without
a prefix at all, then the corresponding C<.pm> file (or another
language's equivalent code module) is searched for in the appropriate
module library path. If found, the code module C<require>'d into the Pod
parser (usually to add a class implementing a particular Pod extension).
If no such code module is found, a suitable C<.pod> file is searched for
instead, the contents parsed as Pod, and the resulting block objects
inserted into the main file's representation.

You can use fully and partially specified module names (as with Perl 6
modules):

    =use Perldoc::Plugin::XHTML-1.2.1-(*)

Any options that are specified after the module name:

    =use Perldoc::Plugin::Image  :Jpeg  prefix=>'http://dev.perl.org'

are passed to the internal C<require> that loads the corresponding module.

Collectively these alternatives allow you to create standard
documentation inserts or stylesheets, to include Pod extracted from
other code files, or to specify new types of documentation blocks and
formatting codes:

=begin item 

To create a standard Pod insertion or stylesheet, create a C<.pod>
file and install it in your documentation path. Load it with either:

=for code :allow<R>
    =use R<Pod::Insertion::Name>

or:

=for code :allow<R>
    =use pod:R<Pod::Insertion::Name>

or:

=for code :allow<R>
    =use file:R</full/path/spec/Pod/Insertion/Name.pod>

or even:

=for code :allow<R>
    =use http://R<www.website.com/Pod/Insertion/Name.pod>

=end item

=begin item 

To insert the Pod from a C<.pm> file (for example, to have your class
documentation include documentation from a base class):

=for code :allow<R>
    =use pod:R<Some::Other::Module>

=end item

=begin item 

To implement a new Pod block type or formatting code, create a C<.pm> file
and load it with either:

=for code :allow<R>
    =use R<New::Perldoc::Subclass>

or (more explicitly):

=for code :allow<R>
    =use perl6:R<New::Perldoc::Subclass>

=end item

=begin item 

To create a module that inserts Pod and also C<require>'s a parser
extension, install a C<.pod> file that contains a nested C<=use> that
imports the necessary plug-in code. Then load the Pod file as above.

A typical example would be a Perldoc extension that also needs to
specify some L<preconfiguration|#Block pre-configuration>:

=for code :allow<R>
    =use R<Hybrid::Content::Plus::Extension>

Then, in the file R<some_perl_doc_dir/Hybrid/Content/Plus/Extension.pod>:

    =begin code :allow<R>
    =comment This file sets some config and also enables the Graph block

    =config Graph :formatted< B >

    =use perl6:Perldoc::Plugin::Graph-(*)-cpan:MEGAGIGA
    =end code

=end item

Note that C<=use> is a fundamental Perldoc directive, like C<=begin> or
C<=encoding>, so there is no paragraph or delimited form of C<=use>.


=head1 SUMMARY

=head2 Directives
=begin table :nested

    Directive           Specifies
    _________           ____________________________________________________
    C<=alias>           Explicitly define an alias
    C<=begin>           Start of an explicitly terminated block
    C<=config>          Lexical modifications to a block or formatting code
    C<=encoding>        Encoding scheme for subsequent text
    C<=end>             Explicit termination of a C<=begin> block
    C<=for>             Start of an implicitly (blank-line) terminated block
    C<=use>             Transclusion of content; loading of a Perldoc module

=end table :nested


=head2 Blocks
=begin table :nested

    Block typename      Specifies
    ______________      ___________________________________________________
    C<=code>            Verbatim pre-formatted sample source code
    C<=comment>         Content to be ignored by all renderers
    C<=head>R<N>        I<N>th-level heading
    C<=input>           Pre-formatted sample input
    C<=item>            First-level list item
    C<=item>R<N>        I<N>th-level list item
    C<=nested>          Nest block contents within the current context
    C<=output>          Pre-formatted sample output
    C<=para>            Ordinary paragraph
    C<=table>           Simple rectangular table
    C<=DATA>            Perl 6 data section
    C<=END>             No ambient blocks after this point
    C<=>R<RESERVED>     Semantic blocks (C<=SYNOPIS>, C<=BUGS>, etc.)
    C<=>R<Typename>     User-defined block

=end table

=head2 Formatting codes
=config C<> :allow<R V>
=begin table :nested

    Formatting code      Specifies
    _______________      ___________________________________________________
    C<A<...>>            Ambient back-reference
    C<B<...>>            Basis/focus of sentence (typically rendered bold)
    C<C<...>>            Code (typically rendered fixed-width)
    C<D<...|...;...>>    Definition (C<D<R<defined term>|R<synonym>;R<synonym>;...>>)
    C<E<...>>            Entity name or numeric codepoint
    C<I<...>>            Important (typically rendered in italics)
    C<K<...>>            Keyboard input (typically rendered fixed-width)
    C<L<...|...>>        Link (C<L<R<display text>|R<destination URI>>>)
    C<M<...:...>>        Module-defined code (C<M<R<scheme>:R<contents>>>)
    C<N<...>>            Note (not rendered inline)
    C<P<...>>            Placement link
    C<V<R><...>>         Replaceable component or metasyntax
    C<S<...>>            Space characters to be preserved
    C<T<...>>            Terminal output (typically rendered fixed-width)
    C<U<...>>            Unusual (typically rendered with underlining)
    C<V<V><...>>         Verbatim (internal formatting codes ignored)
    C<X<...|..,..;...>>  Index entry (C<X<R<display text>|R<entry>,R<subentry>;...>>)
    C<Z<...>>            Zero-width comment (contents never rendered)

=end table

=end pod
