Compose characters in the current region.
Characters are composed relatively, i.e. composed by overstriking
or stacking depending on ascent, descent and other metrics of
For instance, if the region has three characters "XYZ", X is
regarded as BASE glyph, and Y is displayed:
(1) above BASE if Y's descent value is not positive
(2) below BASE if Y's ascent value is not positive
(3) on BASE (i.e. at the BASE position) otherwise
and Z is displayed with the same rule while regarding the whole
XY glyphs as BASE.
When called from a program, expects these four arguments.
First two arguments START and END are positions (integers or markers)
specifying the region.
Optional 3rd argument COMPONENTS, if non-nil, is a character, a string
or a vector or list of integers and rules.
If it is a character, it is an alternate character to display instead
of the text in the region.
If it is a string, the elements are alternate characters. In
this case, TAB element has a special meaning. If the first
character is TAB, the glyphs are displayed with left padding space
so that no pixel overlaps with the previous column. If the last
character is TAB, the glyphs are displayed with right padding
space so that no pixel overlaps with the following column.
If it is a vector or list, it is a sequence of alternate characters and
composition rules, where (2N)th elements are characters and (2N+1)th
elements are composition rules to specify how to compose (2N+2)th
elements with previously composed N glyphs.
A composition rule is a cons of global and new glyph reference point
symbols. See the documentation of `reference-point-alist' for more
Optional 4th argument MODIFICATION-FUNC is a function to call to adjust the composition when it gets invalid because of a change of text in the composition.