const SNo : set prop const minus_SNo : set set term - = minus_SNo axiom SNo_minus_SNo: !x:set.SNo x -> SNo - x const SNoLe : set set prop term <= = SNoLe infix <= 2020 2020 const abs_SNo : set set 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 -> SNo - y -> abs_SNo (x + - z) <= abs_SNo (x + - y) + abs_SNo (y + - z) claim !x:set.!y:set.!z:set.SNo x -> SNo y -> SNo z -> abs_SNo (x + - z) <= abs_SNo (x + - y) + abs_SNo (y + - z)