List of directives to control display of glyphless characters.
Each element has the form (GROUP . METHOD), where GROUP is a
symbol specifying the character group, and METHOD is a symbol
specifying the method of displaying characters belonging to that
group.
GROUP must be one of these symbols:
`c0-control': U+0000..U+001F, but excluding newline and TAB.
`c1-control': U+0080..U+009F.
`format-control': Characters of Unicode General Category `Cf',
such as U+200C (ZWNJ), U+200E (LRM), but
excluding characters that have graphic images,
such as U+00AD (SHY).
`no-font': characters for which no suitable font is found.
For character terminals, characters that cannot
be encoded by `terminal-coding-system'.
METHOD must be one of these symbols:
`zero-width': don't display.
`thin-space': display a thin (1-pixel width) space. On character
terminals, display as 1-character space.
`empty-box': display an empty box.
`acronym': display an acronym of the character in a box. The
acronym is taken from `char-acronym-table', which see.
`hex-code': display the hexadecimal character code in a box.