Specifies the type of function passed to g_tree_traverse.
 It is passed the key and value of each node, together with the user_data parameter passed to g_tree_traverse. 
            If the function returns true, the traversal is stopped.
| key | 
                 a key of a Tree node  | 
            
| value | 
                 the value corresponding to the key  | 
            
| data | 
                 user data passed to   | 
            
| 
                 true to stop the traversal  |