Tcl_Merge

Create a list as a string from a collection of strings, passed in by argc/argv. This used to be the internals of list prior to the introduction of the Tcl_Obj API.

char *Tcl_Merge(int argc, const char *const *argv)

Manual page (8.6, but the API has been effectively static for decades).

If you're using this, seriously consider using Tcl_NewListObj instead.