docs: 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]