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.

def reconstruct_indices(self):
        """
        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.
        """
        del self.i2f, self.f2i
        self.f2i, self.i2f = {}, {}

        for i, w in enumerate(self.words):
            self.f2i[w] = i
            self.i2f[i] = w