code: def subset(self, words):
          
          v = self.__class__(unk=self._unk)
          unique = lambda seq: len(set(seq)) == len(seq)
          assert unique(words)
          for w in words:
              if w in self:
                  v.add(w, count=self.count(w))
          return v
docs: Get a new Vocab containing only the specified subset of words.
  
          If w is in words, but not in the original vocab, it will NOT be in the subset vocab.
          Indices will be in the order of `words`. Counts from the original vocab are preserved.
  
          :return (Vocab): a new Vocab object