compose-string-internal is a built-in function in `C source code'.

(compose-string-internal STRING START END &optional COMPONENTS MODIFICATION-FUNC)

Internal use only.

Compose text between indices START and END of STRING, where
START and END are treated as in `substring'. Optional 4th
and 5th arguments are COMPONENTS and MODIFICATION-FUNC
for the composition. See `compose-string' for more details.