| 1 | 5. i: Vertices(the_graph) 6. (i nil) (inl(i) nil) (inl(i) s) (inr(i) s) | 1 step |
|   |
| 2 | l_disjoint(Vertices(the_graph)+Vertices(the_graph);nil;s) | 1 step |
|   |
| 3 | no_repeats(Vertices(the_graph)+Vertices(the_graph);nil) | 1 step |
|   |
| 4 | paren(Vertices(the_graph);nil) | 1 step |