code: def fts_intersection(self, segs):
          
          fts_vecs = [self.fts(s) for s in self.filter_segs(segs)]
          return reduce(lambda a, b: a & b, fts_vecs)
docs: Return the features shared by `segs`
  
          Args:
              segs (list): list of Unicode IPA segments
  
          Returns:
              set: set of (value, feature) tuples shared by the valid segments in
                   `segs`