const ordinal : set prop const SNo : set prop axiom ordinal_SNo: !x:set.ordinal x -> SNo x const In : set set prop term iIn = In infix iIn 2000 2000 const SNoLt : set set prop term < = SNoLt infix < 2020 2020 lemma !x:set.!y:set.ordinal x -> y iIn x -> ordinal y -> SNo y -> y < x var x:set var y:set hyp ordinal x hyp y iIn x claim ordinal y -> y < x