<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def evaluate(self, gold):
 """Evaluate the accuracy of this tagger using a gold standard corpus. :param list(list(tuple(str, str))) gold: The list of tagged sentences to score the tagger on. :returns: Tagger accuracy value. :rtype: float """

        tagged_sents = self.tag_sents([w for (w, t) in sent] for sent in gold)
        gold_tokens = sum(gold, [])
        test_tokens = sum(tagged_sents, [])
        accuracy = float(sum(x == y for x, y in six.moves.zip(gold_tokens, test_tokens))) / len(test_tokens)
        return accuracy