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 SNoLe : set set prop term <= = SNoLe infix <= 2020 2020 const abs_SNo : set set lemma !x:set.!y:set.SNo x -> SNo y -> SNo (x + y) -> SNo - x -> abs_SNo (x + y) <= abs_SNo x + abs_SNo y var x:set var y:set hyp SNo x hyp SNo y claim SNo (x + y) -> abs_SNo (x + y) <= abs_SNo x + abs_SNo y