The less than or equal relation.
Source: The Art of Prolog
Program source code: lesseq_operator.pl
/* X lesseq Y :- X and Y are natural numbers, such that X is less than or equal to Y. We use lesseq to represent the operator rather than cause problems with an error message from Prolog about redefining an operator! */ :- op(40,xfx, 'lesseq'). 0 lesseq X :- natural_number(X). s(X) lesseq s(Y) :- X lesseq Y. natural_number(0). natural_number(s(X)) :- natural_number(X). % Program 3.2: The less than or equal relation