FcConfigSubstituteWithPat (3) - Linux Manuals
FcConfigSubstituteWithPat: Execute substitutions
FcConfigSubstituteWithPat - Execute substitutions
Performs the sequence of pattern modification operations, if kind is FcMatchPattern, then those tagged as pattern operations are applied, else if kind is FcMatchFont, those tagged as font operations are applied and p_pat is used for <test> elements with target=pattern. Returns FcFalse if the substitution cannot be performed (due to allocation failure). Otherwise returns FcTrue. If config is NULL, the current configuration is used.