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 lemma !x:set.!y:set.!z:set.SNo x -> SNo y -> SNo z -> x + y = x + z -> SNo - x -> y = z claim !x:set.!y:set.!z:set.SNo x -> SNo y -> SNo z -> x + y = x + z -> y = z