Compose characters in string STRING.
The return value is STRING with the `composition' property put on all
the characters in it.
Optional 2nd and 3rd arguments START and END specify the range of
STRING to be composed. They default to the beginning and the end of
STRING respectively.
Optional 4th argument COMPONENTS, if non-nil, is a character or a
sequence (vector, list, or string) of integers. See the function
`compose-region' for more detail.
Optional 5th 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.