The one_to_one_equal example only works for sets, it looks like. This is a popular example in the Prolog literature, where they say sort(A, B) :- permute(A

The one_to_one_equal example only works for sets, it looks like. This is a popular example in the Prolog literature, where they say sort(A, B) :- permute(A, B), ascending(B). (permute corresponds to your one_to_one_equal.)