const SNo : set prop const minus_SNo : set set term - = minus_SNo axiom SNo_minus_SNo: !x:set.SNo x -> SNo - x const SNoLt : set set prop term < = SNoLt infix < 2020 2020 axiom minus_SNo_Lt_contra: !x:set.!y:set.SNo x -> SNo y -> x < y -> - y < - x axiom minus_SNo_invol: !x:set.SNo x -> - - x = x claim !x:set.!y:set.SNo x -> SNo y -> - x < y -> - y < x