The goal fail always fails.
Examples
?- fail.
no a(1).
a(2).
a(3).
test :- a(X), a(Y), write(Y), write(' '), write(X), nl, fail.
?- test.
1 1
2 1
3 1
1 2
2 2
3 2
1 3
2 3
3 3
nofail - always fails.The goal fail always fails.
?- fail.
no a(1).
a(2).
a(3).
test :- a(X), a(Y), write(Y), write(' '), write(X), nl, fail.
?- test.
1 1
2 1
3 1
1 2
2 2
3 2
1 3
2 3
3 3
no