====== Factorial ======
{{tag>math recursion}}
===== Description =====
Factorial


**Source**:  The Art of Prolog
===== Download =====
Program source code: {{factorial.pl}}
===== Listing =====
<code prolog>
/*

	factorial(N,F) :- F equals N factorial.

*/



	factorial(0,s(0)).

	factorial(s(N),F) :- factorial(N,F1), times(s(N),F1,F).



	times(0,X,0).

	times(s(X),Y,Z) :- times(X,Y,XY), plus(XY,Y,Z).



	plus(0,X,X) :- natural_number(X).

	plus(s(X),Y,s(Z)):- plus(X,Y,Z).



	natural_number(0).

	natural_number(s(X)) :- natural_number(X).



%	Program 3.6: Computing fa
ctorials

</code>
===== Comments =====
RTERR: No permission to redefine built-in predicate 'plus/3'