code: def fts_count(self, fts, inv):
          
          return len(list(filter(lambda s: self.fts_match(fts, s), inv)))
docs: Return the count of segments in an inventory matching a given
          feature mask.
  
          Args:
              fts (set): feature mask given as a set of (value, feature) tuples
              inv (set): inventory of segments (as Unicode IPA strings)
  
          Returns:
              int: number of segments in `inv` that match feature mask `fts`