const CSNo : set prop const mul_CSNo : set set set axiom CSNo_mul_CSNo: !x:set.!y:set.CSNo x -> CSNo y -> CSNo (mul_CSNo x y) lemma !x:set.!y:set.CSNo x -> CSNo y -> CSNo (mul_CSNo x y) -> CSNo (mul_CSNo y x) -> mul_CSNo x y = mul_CSNo y x var x:set var y:set hyp CSNo x hyp CSNo y claim CSNo (mul_CSNo x y) -> mul_CSNo x y = mul_CSNo y x