Negation by failure, if the goal succeeds then failing starts else succeeds. It is a predefined prefix (FY) operator with predence 900. It is better to use this operator then not/1 because this is operator is defined by the ISO Prolog standard.
see also: not/1 Prolog operators
| Examples | |
| \+ fail. | succeeds |
| \+(X=1 ; X=2), X=3. | fails |