human
Solve the following python programming problem given the following code:
def backref_listeners(attribute, key, uselist) : 
    'Apply listeners to synchronize a two-way relationship.' 
    parent_token = attribute.impl.parent_token 
    parent_impl = attribute.impl 
    def _acceptable_key_err(child_state, initiator, child_impl) : 
        raise ValueError (('Bidirectional attribute conflict detected: Passing object %s to attribute "%s" triggers a modify event on attribute "%s" via the backref "%s".' % (state_str (child_state), initiator.parent_token, child_impl.parent_token, attribute.impl.parent_token))) 
def emit_backref_from_scalar_set_event(state, child, oldchild, initiator) : 
        if (oldchild is child) : 
            return child 
if ((oldchild is not None) and (oldchild is not PASSIVE_NO_RESULT) and (oldchild is not NEVER_SET)) : 
            (old_state, old_dict) = (instance_state (oldchild), instance_dict (oldchild)) 
            impl = old_state.manager [key].impl 
            if ((initiator.impl is not impl) or (initiator.op not in (OP_REPLACE, OP_REMOVE))) : 
                impl.pop (old_state, old_dict, state.obj (), parent_impl._append_token, passive = PASSIVE_NO_FETCH) 
if (child is not None) : 
            (child_state, child_dict) = (instance_state (child), instance_dict (child)) 
            child_impl = child_state.manager [key].impl 
            if ((initiator.parent_token is not parent_token) and (initiator.parent_token is not child_impl.parent_token)) : 
                _acceptable_key_err (state, initiator, child_impl) 
else : 
                if ((initiator.impl is not child_impl) or (initiator.op not in (OP_APPEND, OP_REPLACE))) : 
                    child_impl.append (child_state, child_dict, state.obj (), initiator, passive = PASSIVE_NO_FETCH) 
return child 
def emit_backref_from_collection_append_event(state, child, initiator) : 
        if (child is None) : 
            return 
(child_state, child_dict) = (instance_state (child), instance_dict (child)) 
        child_impl = child_state.manager [key].impl 
        if ((initiator.parent_token is...

    
    

    Fix the buggy line: event.listen (parent_token, 'set', emit_backref_from_scalar_set_event, retval = True, raw = True)
assistant
#Here is Python code to fix the buggy line: event.

event.listen (attribute, 'set', emit_backref_from_scalar_set_event, retval = True, raw = True)