code: def reconstruct_indices(self):
          
          del self.i2f, self.f2i
          self.f2i, self.i2f = {}, {}
  
          for i, w in enumerate(self.words):
              self.f2i[w] = i
              self.i2f[i] = w
docs: Reconstruct word indices in case of word removals.
          Vocabulary does not handle empty indices when words are removed,
            hence it need to be told explicity about when to reconstruct them.