/*
   factorial(N,F) :- F is the integer N factorial.
*/ 
     factorial(N,F) :- factorial(0,N,1,F).

     factorial(I,N,T,F) :-
        I < N, I1 is I+1, T1 is T*I1, factorial(I1,N,T1,F).
     factorial(N,N,F,F). 

%  Program 8.3   An iterative factorial