====== Negation ======
{{tag>operators}}
===== Description =====
Negation as failure.


**Source**:  The Art of Prolog
===== Download =====
Program source code: {{negation.pl}}

===== Listing =====
<code prolog>
/*

   not X :- X is not provable.

*/

	:- op(900, fx, [not]).



     not X :- X, !, fail.

     not _.



%  Program 11.6   Negation as failur



</code>
===== Comments =====