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

(compose-chars &rest ARGS)

Return a string from arguments in which all characters are composed.
For relative composition, arguments are characters.
For rule-based composition, Mth (where M is odd) arguments are
characters, and Nth (where N is even) arguments are composition rules.
A composition rule is a cons of glyph reference points of the form
(GLOBAL-REF-POINT . NEW-REF-POINT). See the documentation of
`reference-point-alist' for more detail.