Factorial of n is n!= 1 * 2* 3 * 4 *... * n   
		or
			0! = 1
			n! = (n-1)! * n        

		?- fact(4,R).
		R = 24

		fact(0,1).
		fact(N,R):- fact(N1,R1),N is N1 + 1,R is R1 * N.

		Written with a collecting variable.
		
		?- fact(4,R).
		R = 24

                fact(N,F):- fact1(N,1,F).

		fact1(0,F,F).
		fact1(N,X,F):- N is M + 1, Y is X * N, fact1(M,Y,F).