code: def get_lemma_by_id(self, mongo_id):
          '''
          Builds a Lemma object from the database entry with the given
          ObjectId.
  
          Arguments:
          - `mongo_id`: a bson.objectid.ObjectId object
          '''
          cache_hit = None
          if self._lemma_cache is not None:
              cache_hit = self._lemma_cache.get(mongo_id)
          if cache_hit is not None:
              return cache_hit
          lemma_dict = self._mongo_db.lexunits.find_one({'_id': mongo_id})
          if lemma_dict is not None:
              lemma = Lemma(self, lemma_dict)
              if self._lemma_cache is not None:
                  self._lemma_cache.put(mongo_id, lemma)
              return lemmf get_lemma_by_id(self, mongo_id):
          '''
          Builds a Lemma object from the database entry with the given
          ObjectId.
  
          Arguments:
          - `mongo_id`: a bson.objectid.ObjectId object
          '''
          cache_hit = None
          if self._lemma_cache is not None:
              cache_hit = self._lemma_cache.get(mongo_id)
          if cache_hit is not None:
              return cache_hit
          lemma_dict = self._mongo_db.lexunits.find_one({'_id': mongo_id})
          if lemma_dict is not None:
              lemma = Lemma(self, lemma_dict)
              if self._lemma_cache is not None:
                  self._lemma_cache.put(mongo_id, lemma)
              return lemma
docs: Builds a Lemma object from the database entry with the given
          ObjectId.
  
          Arguments:
          - `mongo_id`: a bson.objectid.ObjectId object