%Source: Sterling, Shapiro - The Art of Prolog. Program 8.1 - Program 8.12.
%query:factorial(g,f).
factorial(N,F) :- factorial(N,1,F).
factorial(N,T,F) :- N > 0, T1 is T * N, N1 is N - 1, factorial(N1, T1, F).
factorial(0,F,F).