% Here is another version that does error checking once at the
% start, rather than on every recursion.

factorial2(N, F) :-
	integer(N),
	fact2(N, F).

fact2(1, 1).
fact2(N, F) :-
	N > 1,
	M is N-1,
	fact2(M, F2),
	F is N*F2.