#include <fontconfig/fontconfig.h>
FcBool FcCharSetMerge (FcCharSet *a, const FcCharSet *b, FcBool *changed);

Adds all chars in b to a. In other words, this is an in-place version of FcCharSetUnion. If changed is not NULL, then it returns whether any new chars from b were added to a. Returns FcFalse on failure, either when a is a constant set or from running out of memory.