code: def lemma(lemma_key):
      
      if lemma_key in LEMMAS_DICT:
          return LEMMAS_DICT[lemma_key]
      split_lemma_key = lemma_key.split('.')
  
      synset_key = '.'.join(split_lemma_key[:3])
      lemma_literal = split_lemma_key[3]
  
      lemma_obj = Lemma(synset_key,lemma_literal)
      LEMMAS_DICT[lemma_key] = lemma_obj
      return lemma_obj
docs: Returns the Lemma object with the given key.
  
      Parameters
      ----------
      lemma_key : str
        Key of the returned lemma.
  
      Returns
      -------
      Lemma
        Lemma matching the `lemma_key`.