const In : set set prop term iIn = In infix iIn 2000 2000 term nIn = \x:set.\y:set.~ x iIn y term Subq = \x:set.\y:set.!z:set.z iIn x -> z iIn y axiom FalseE: ~ False const Empty : set axiom Empty_Subq_eq: !x:set.Subq x Empty -> x = Empty claim !x:set.(!y:set.nIn y x) -> x = Empty