twee-0.1: tests/robbins2.p

cnf(a, axiom, '+'(X, Y) = '+'(Y, X)).
cnf(a, axiom, '+'(X, '+'(Y, Z)) = '+'('+'(X, Y), Z)).
cnf(a, axiom, '-'('+'('-'('+'(X, Y)), '-'('+'(X, '-'(Y))))) = X).
cnf(a, axiom, '-'('-'(a)) != a).