const In : set set prop term iIn = In infix iIn 2000 2000 term nIn = \x:set.\y:set.~ x iIn y const Sing : set set axiom SingE: !x:set.!y:set.y iIn Sing x -> y = x const Inj1 : set set const Empty : set axiom Inj1NE1: !x:set.Inj1 x != Empty claim !x:set.~ Inj1 x iIn Sing Empty