Compose characters in current buffer after position POS.
It looks up the char-table `composition-function-table' (which
see) by a character at POS, and compose characters after POS
according to the contents of `composition-function-table'.
Optional 2nd arg LIMIT, if non-nil, limits characters to compose.
Optional 3rd arg OBJECT, if non-nil, is a string that contains the
text to compose. In that case, POS and LIMIT index into the string.
This function is the default value of `compose-chars-after-function'.