code: def lemma(self, verb, parse=True):
          
          if dict.__len__(self) == 0:
              self.load()
          if verb.lower() in self._inverse:
              return self._inverse[verb.lower()]
          if verb in self._inverse:
              return self._inverse[verb]
          if parse is True: # rule-based
              return self.find_lemma(verb)
docs: Returns the infinitive form of the given verb, or None.