Compose - X client mappings for multi-key input sequences
The X library, libX11, provides a simple input method for characters beyond
  those represented on typical keyboards using sequences of key strokes that are
  combined to enter a single character.
The compose file is searched for in the following order:
  - -
- If the environment variable $XCOMPOSEFILE is set, its value is used
      as the name of the Compose file.
- -
- If the user's home directory has a file named .XCompose, it is used
      as the Compose file.
- -
- The system provided compose file is used by mapping the locale to a
      compose file from the list in
      /usr/X11R7/lib/X11/locale/compose.dir.
Compose files can use an "include" instruction.
    This allows local modifications to be made to existing compose files without
    including all of the content directly. For example, the system's iso8859-1
    compose file can be included with a line like this:
include
  "%S/iso8859-1/Compose"
There are several substitutions that can be made in the file name
    of the include instruction:
  - %H
- expands to the user's home directory (the $HOME environment
      variable)
- %L
- expands to the name of the locale specific Compose file (i.e.,
      "/usr/X11R7/lib/X11/locale/<localename>/Compose")
- %S
- expands to the name of the system directory for Compose files (i.e.,
      "/usr/X11R7/lib/X11/locale")
For example, you can include in your compose file the default
    Compose file by using:
include "%L"
and then rewrite only the few rules that you need to change. New compose rules
  can be added, and previous ones replaced.
Compose files are plain text files, with a separate line for each compose
  sequence. Comments begin with # characters. Each compose sequence
  specifies one or more events and a resulting input sequence, with an optional
  comment at the end of the line:
EVENT [EVENT...] : RESULT
  [# COMMENT]
Each event consists of a specified input keysym, and optional
    modifier states:
[([!] ([~] MODIFIER)...) |
  None] <keysym>
If the modifier list is preceded by "!" it must
    match exactly. MODIFIER may be one of Ctrl, Lock, Caps, Shift, Alt or Meta.
    Each modifier may be preceded by a "~" character to
    indicate that the modifier must not be present. If "None"
    is specified, no modifier may be present.
The result specifies a string, keysym, or both, that the X client
    receives as input when the sequence of events is input:
"STRING" | keysym |
  "STRING" keysym
Keysyms are specified without the XK_ prefix.
Strings may be direct text encoded in the locale for which the
    compose file is to be used, or an escaped octal or hexadecimal character
    code. Octal codes are specified as "\123" and hexadecimal
    codes as "\x3a". It is not necessary to specify in the
    right part of a rule a locale encoded string in addition to the keysym name.
    If the string is omitted, Xlib figures it out from the keysym according to
    the current locale. I.e., if a rule looks like:
<dead_grave> <A> : "\300"
  Agrave
the result of the composition is always the letter with the "\300"
  code. But if the rule is:
<dead_grave> <A> : Agrave
the result depends on how Agrave is mapped in the current locale.
  - XCOMPOSEFILE
- File to use for compose sequences.
- XCOMPOSECACHE
- Directory to use for caching compiled compose files.
  - $HOME/.XCompose
- User default compose file if XCOMPOSEFILE is not set.
- /usr/X11R7/lib/X11/locale/compose.dir
- File listing the compose file path to use for each locale.
- /usr/X11R7/lib/X11/locale/<localemapping>/Compose
- System default compose file for the locale, mapped via compose.dir.
- /var/cache/libx11/compose/
- System-wide cache directory for compiled compose files.
- $HOME/.compose-cache/
- Per-user cache directory for compiled compose files.
XLookupString(3), XmbLookupString(3), XwcLookupString(3),
  Xutf8LookupString(3), mkcomposecache(1), locale(7).
Xlib - C Language X Interface