'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'

def entities(self, subject_id):

    res = self._cache.get(subject_id)
    if (not res):
        raise KeyError('No    such    subject')
    else:
        return res
