const Sep : set (set prop) set const nIn : set set prop term setminus = \x:set.\y:set.Sep x \z:set.nIn z y const In : set set prop term iIn = In infix iIn 2000 2000 axiom SepI: !x:set.!p:set prop.!y:set.y iIn x -> p y -> y iIn Sep x p claim !x:set.!y:set.!z:set.z iIn x -> nIn z y -> z iIn setminus x y