Set::Infinite TODO 

    - verify POD format and contents; mix POD and program

BACKTRACK

	make backtrack_result method

	test backtrack on:
		iterate
		compact
		no_cleanup

	change $backtrack_arg2 = $arg->quantize(@param)->union(); to span()

	backtracking select - see '	# (TODO) ???? ' in Infinite.pm, backtrack.t

	backtracking: should be extensible. Use tables instead of if-else
	backtracking: change ->{method} to sub-pointer instead of string ?
	backtracking: define 'backwards' method instead of if-else (near 3-08)

	backtracking: define union, complement, contains...

	backtracking: implement remaining tests in backtrack.t
	verify results in backtrack.t

	ok - backtracking quantize should round up and down values, according to unit, quant.
		-- check if this is really necessary!
		-- try using span, or create result directly (faster)
		-- or: use 'offset' instead
	backtracking select should move up and down values, according to 'by' limits.
	See: backtrack.t in 'partially null "date"'

	backtracking: document backtracking resolution; document internal object data (partially done)

SYNTAX

	remove iterate ?
	'iterate' options 'no-sort', 'no-merge', 'keep-null' ...

	sort by[] -- 0 1 2 ... freq-3 freq-2 freq-1

	ical.pl: enter data as epoch; translate on exit.
	use DTSTART as default hour, minute, ...

	extra: wrappers (like: weekday('thursday') )
	some tough cases: yearweek(10) monthweek(2)
	-- partly done: ICalSet.pm

	check: RFC2445 support completeness

	ical.pl: BYWEEKNO / WKST
	ical.pl: test bysetpos
	ical.pl: check behaviour when mixing rule parts (BYWEEKNO + BYYEARDAY);
	-- see end of 4.3.10 in RFC2445
	-- check how to integrate with DTSTART
	ical.pl: put UNTIL inside rrule
	ical.pl: where do DTSTART", "DTEND", "DUE", "EXDATE" and "RDATE" belong?
	better DURATION
	TZID

	This looks wrong - is it?
	   "FREQ=YEARLY;BYMONTH=1,2" increases the number of days within the
	   yearly recurrence set from 1 (if BYMONTH tag is not present) to 2.

	'select' switch for negative-index overflow behaviour (end of subset, or end of freq)
	tests: add test for negative 'by'

	negative values in select should count backwards from end
	-- test! insert debug -- ical.pl 134 should show 2001-09-30

	remove 'interval' option from select

	old: correct syntax problems due to accepting ( [ 1 .. 10 ] ) as input - see POD.

	make 'freq' work with 'days', etc.

	extra: make offset accept ICal times

	extra: offset option "middle"

	extra: "indexes" filter to help dealing with sparse lists:
	(,,10,,12,,14,,,)->indexes gives (2,4,6)

	extra: Make a 'strict' option for "quantize" and other filters, 
	that will intersect
	each unit back to the object, that is:
	Interval:                     [************************]
	Quantized:                [*********)[*********)[*********)
	Quantized->Stricted:          [*****)[*********)[******]

	think about: "push-pop", "dup", "drop" mechanism

	think about: some "if-else" mechanism

TESTS

	make t/icalset.t

	old: fix Bigfloat tests

	put 't/element_inf.t' back

DOCUMENTATION

	check: POD formatting 

	verify, document parameter syntax everywhere

	document:
		new module 'ICalSet'
		new method 'compact'
		offset option 'mode=>circle'
		new method 'list'
		iterate
		no_cleanup

	document '->trace({options})->' method

	document: quantize extensibility through hash-subs

CACHEING

	cache quantize by month/day/...
	-- doesn't work if 'quant' is changed
	-- doesn't check 'intersects'

	make a 'flush'-cache method. Make a 'cache-registry'.

	think about: caching for union, etc  

	old: Create a `dirty' variable so it knows when to cleanup.

	extra: caching for the filters. 

OTHER

	find a better (bigger span) gmtime/timegm

	check for negative-epoch errors (quantize, select, offset).

	remove a lot of debug #-print lines

	internal: it may be good to have a pointer to the "root" object
	in the filters. They already have a "parent" pointer.

	oo: provide a general filter "install" mechanism (quantize, select, etc)
	check: cleanup "type" hack

	extra: syntax for quantize: ...->quantize('years')

	old: Make a global mode for `open_*' 

	old: Quantize_Date should care of daylight savings time

NOT-TODO

	Keep a pointer to object in "quantize", such that, if the object is changed,
	"quantize" will re-initialize. 
	Make a "object_modified" flag in "quantize",
	that the object should point back to. Sort of a signal handler.

