foo((pred((A :: in), (B :: out)) is det :- p(A, C), q(C, B)), D, E).
{(a(1), X = 1, (b, Y = 1, (c, Z = 1 ; Z = 2) ; Y = 2, Z = 2) ; X = 2, Y = 2, Z = 2), A = 1}.
[(a(1), X = 1 & X = 2, Y = 2), A = 1].
[(a :: m), (b :: n)].
[foo, (promise_pure), bar].
bound((ground ; ground - unique)).