code: def fts_match(self, fts_mask, segment):
          
          fts_mask = set(fts_mask)
          fts_seg = self.fts(segment)
          if fts_seg:
              return fts_seg <= fts_mask
          else:
              return None
docs: Evaluates whether a set of features 'match' a segment (are a subset
          of that segment's features)
  
          Args:
              fts_mask (list): list of (value, feature) tuples
              segment (unicode): IPA string corresponding to segment (consonant or
                                 vowel)
          Returns:
              bool: None if `segment` cannot be parsed; True if the feature values
                    of `fts_mask` are a subset of those for `segment`