const In : set set prop term iIn = In infix iIn 2000 2000 term Subq = \x:set.\y:set.!z:set.z iIn x -> z iIn y term nIn = \x:set.\y:set.~ x iIn y claim !x:set.!y:set.!z:set.Subq x y -> nIn z y -> ~ z iIn x