op(Precedence, Associativity, Operator)
Declares operators. Operators declared inside entities 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(200, fy, +). :- op(200, fy, ?). :- op(200, fy, @). :- op(200, fy, -).