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These predicates fail if Graph is cyclic. If Graph is not connected, the sub-graphs are individually sorted, where the root of each subgraph is in the first layer, the nodes connected to the roots in the second, etc.
?- top_sort([1-, 2-, 3-], L). L = [1, 2, 3]
top_sort(Graph, Tail, Sorted) :- top_sort(Graph, Sorted0), append(Sorted0, Tail, Sorted).
The original version returned all vertices in a layer in reverse order. The current one returns them in standard order of terms, i.e., each layer is an ordered set.