Moves all matching files from the set *unmatched* to the set
        *matched*.

        Both *matched* and *unmatched* are sets of string, the strings
        being unqualified file names

def match_files(self, matched, unmatched):
        """Moves all matching files from the set *unmatched* to the set
        *matched*.

        Both *matched* and *unmatched* are sets of string, the strings
        being unqualified file names"""
        this_match = set(self.file_filter(unmatched))
        matched |= this_match
        unmatched -= this_match