code: def extract_tweets(json_file):
      
      for screen_name, tweet_iter in groupby(parse_json(json_file), lambda x: x[0]):
          tweets = [t[1] for t in tweet_iter]
          yield screen_name, ' '.join(tweets)
docs: Yield screen_name, string tuples, where the string is the
      concatenation of all tweets of this user.