const In : set set prop term iIn = In infix iIn 2000 2000 term nIn = \x:set.\y:set.~ x iIn y const Empty : set axiom EmptyE: !x:set.nIn x Empty const ordsucc : set set lemma !x:set.Empty = ordsucc x -> ~ ~ x iIn ordsucc x claim !x:set.Empty != ordsucc x