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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def stem(self, form, tag):
 """Returns the stem of word with specific form and part-of-speech tag according to the Stanford lemmatizer. Lemmas are cached."""

        key = (form, tag)
        if key not in self.lemma_cache:
            lemma = self.stemmer(*key).word()
            self.lemma_cache[key] = lemma
        return self.lemma_cache[key]