% Factorial in prolog

f(1,1).
f(N,_):-N=<0,fail.
f(N,F):-N>1, write(N),  write('! = ?'), nl,
	N1 is N-1, write('   N1='), write(N1), nl,
	f(N1,F1),
        write(' F is '), write(F1), write(' * '), write(N), nl,
	F is F1 * N,			
	write(N),write('! = '), write(F), nl.