code: def match_files(self, matched, unmatched):
          
          this_match = set(self.file_filter(unmatched))
          matched |= this_match
          unmatched -= this_match
docs: 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