human
Solve the following python programming problem given the following code:
def _watch_group(self, group_id) : 
    get_members_req = self.get_members (group_id) 
    def on_children_change(children) : 
        if (not self._has_hooks_for_group (group_id)) : 
            return False 
children = set (children) 
        last_children = self._group_members [group_id] 
        for member_id in (children - last_children) : 
            hooks = copy.copy (self._hooks_join_group [group_id]) 
            self._watchers.append ((lambda : hooks.run (coordination.MemberJoinedGroup (group_id, utils.to_binary (member_id))))) 
for member_id in (last_children - children) : 
            hooks = copy.copy (self._hooks_leave_group [group_id]) 
            self._watchers.append ((lambda : hooks.run (coordination.MemberLeftGroup (group_id, utils.to_binary (member_id))))) 
self._group_members [group_id] = children 
self._group_members [group_id] = get_members_req.get () 
    try : 
        self._coord.ChildrenWatch (self._path_group (group_id), <MASK>) 
except exceptions.NoNodeError : 
        raise coordination.GroupNotCreated (group_id) 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: get_members_req, self, group_id
assistant
#Here is a proper replacement for <MASK>.
def on_children_change(