gistfile1.txt

nat(0).
nat(X1) :- nat(X), X1 is X + 1.

pair_of_nats(X, Y) :- nat(X), nat(Y).

(0 step)
pair_of_nats(X, Y).

(1 step)
nat(X), nat(Y).

(2 step)
[X=0, Y=0.] (resolved)
X=X1, nat(X2), X1 is X2 + 1, Y=0.
X=0, Y=Y1, nat(Y2), Y1 is Y2 + 1.
X=X1, nat(X2), X1 is X2 + 1, Y=Y1, nat(Y2), Y1 is Y2 + 1.

(3 step)
X=X1, X2=0, X1 is X2 + 1, Y=0. -> [X=1, Y=0.] (resolved)
X=X1, X2=X3, nat(X4), X3 is X4 + 1, X1 is X2 + 1, Y=0.
X=0, Y=Y1, Y2=0, Y1 is Y2 + 1. -> [X=0, Y=1.] (resolved)
X=0, Y=Y1, Y2=Y3, nat(Y4), Y3 is Y4 + 1, Y1 is Y2 + 1.
....