op(Precedence, Associativity, Operator)
Declares operators. Operators declared inside objects and categories have local scope. Global operators can be declared inside a source file by writing the respective directives before the entity opening directives.
op(+integer, +associativity, +atom_or_atom_list)
:- op(950, fx, +). :- op(950, fx, ?). :- op(950, fx, @). :- op(950, fx, -).