is mentioned by

Thm* For any graph the_obj:GraphObject(the_graph). (non-trivial-loop-free(the_graph) topsorted(the_graph;topsort(the_obj))) & (i:V. (i topsort(the_obj))) & no_repeats(V;topsort(the_obj))

[topsort-properties]

Try larger context: Graphs