const SNo : set prop const minus_SNo : set set term - = minus_SNo axiom SNo_minus_SNo: !x:set.SNo x -> SNo - x const add_SNo : set set set term + = add_SNo infix + 2281 2280 const Empty : set lemma !x:set.SNo x -> SNo - x -> x + - x = Empty claim !x:set.SNo x -> x + - x = Empty