docs: def tokenize_fielded(self, tw):
          """ Tokenize tweet and prepend field id before each token.
          >>> t = Tokenizer(fields=['text', 'user.name', 'user.location'])
          >>> list(t.tokenize_fielded(Tweet(json.loads(data.test_tweet))))
          ['text___if', 'text___only', "text___bradley's", 'text___arm', 'text___was', 'text___longer', 'text___.', 'text___best', 'text___photo', 'text___ever', 'text___.', 'text___HASHTAG_oscars', 'text___THIS_IS_A_URL', 'user.name___ellen', 'user.name___degeneres']
          """
          toks = self.tokenize(tw)
          for field, tokens in sorted(toks.items()):
              for tok in tokens:
                  yield '%s___%s' % (field, tok)