:- op(650, xfy, xor).    % exclusive or
:- op(650, xfy, eqv).	 % equivalence
:- op(640, xfy, imp).    % implication
:- op(630, xfy, or).     % disjunction
:- op(620, xfy, and).    % conjunction
:- op(610, fy,  neg).    % negation