const SNo : set prop const add_SNo : set set set term + = add_SNo infix + 2281 2280 axiom SNo_add_SNo: !x:set.!y:set.SNo x -> SNo y -> SNo (x + y) const minus_SNo : set set term - = minus_SNo const abs_SNo : set set lemma !x:set.!y:set.SNo x -> SNo y -> SNo - x -> SNo - y -> SNo (y + - x) -> abs_SNo (x + - y) = abs_SNo (y + - x) var x:set var y:set hyp SNo x hyp SNo y hyp SNo - x claim SNo - y -> abs_SNo (x + - y) = abs_SNo (y + - x)