docs: def tokens(self, si, k):
          '''`si` is a stream item and `k` is a key in this feature. The purpose
          of this method is to dereference the token pointers with
          respect to the given stream item. That is, it translates each
          sequence of token pointers to a sequence of `Token`.
  
          '''
  
          for tokens in self[k]:
              yield [si.body.sentences[tagid][sid].tokens[tid]
                     for tagid, sid, tid in tokens]