code: def entities(self, subject_id):
          
          res = self._cache.get(subject_id)
          if not res:
              raise KeyError("No such subject")
          else:
              return res
docs: Returns all the entities of assertions for a subject, disregarding
          whether the assertion still is valid or not.
  
          :param subject_id: The identifier of the subject
          :return: A possibly empty list of entity identifiers