code: def entities(self, subject_id):
          
          try:
              return [i["entity_id"] for i in self._cache.find({"subject_id":
                      subject_id})]
          except ValueError:
              return []
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