compose-chars-after is a compiled Lisp function in `composite.el'.

(compose-chars-after POS &optional LIMIT OBJECT)

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'.