twee-2.3: tests/minus.p

cnf(plus_zero, axiom,
    '0' + X = X).
cnf(plus_zero, axiom,
    X + '0' = X).
cnf(minus_minus, axiom,
    - -X = X).
cnf(minus_plus, axiom,
    -(X + Y) = -X + -Y).
cnf(goal, conjecture,
    -'0' = '0').