Returns a vocabulary with the most frequent `k` words.

    Args:
      k (integer): specifies the top k most frequent words to be returned.

def most_frequent(self, k):
    """ Returns a vocabulary with the most frequent `k` words.

    Args:
      k (integer): specifies the top k most frequent words to be returned.
    """
    word_count = {w:self.word_count[w] for w in self.words[:k]}
    return CountedVocabulary(word_count=word_count)