docs: def vectorize(json_file, vec, dofit=True):
      """ Return a matrix where each row corresponds to a Twitter account, and
      each column corresponds to the number of times a term is used by that
      account. """
      ## CountVectorizer, efficiently.
      screen_names = [x[0] for x in extract_tweets(json_file)]
      if dofit:
          X = vec.fit_transform(x[1] for x in extract_tweets(json_file))
      else:
          X = vec.transform(x[1] for x in extract_tweets(json_file))
      return screen_names, X