human
Solve the following python programming problem given the following code:
def _event_fixture(self) : 
    session = create_session () 
    @ event.listens_for (session, 'pending_to_persistent') 
    @ event.listens_for (session, 'deleted_to_persistent') 
    @ event.listens_for (session, 'detached_to_persistent') 
    @ event.listens_for (session, 'loaded_as_persistent') 
    def strong_ref_object(sess, instance) : 
        if ('refs' not in sess.info) : 
            sess.info ['refs'] = refs = set () 
else : 
            refs = sess.info ['refs'] 
refs.add (instance) 
@ event.listens_for (session, 'persistent_to_detached') 
    @ event.listens_for (session, 'persistent_to_deleted') 
    @ event.listens_for (session, 'persistent_to_transient') 
    def deref_object(sess, instance) : 
        sess.info ['refs'].discard (instance) 
def prune() : 
        if ('refs' not in session.info) : 
            return 0 
sess_size = len (session.identity_map) 
        session.info ['refs'].clear () 
        gc_collect () 
        session.info ['refs'] = set ((s.obj () for s in session.identity_map.all_states ())) 
        return (sess_size - len (session.identity_map)) 
return (session, self) 

    
    

    Fix the buggy line: return (session, self)
assistant
#Here is Python code to fix the buggy line: return (session, self)
.

return (session, def prune()