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 const setminus : set set set axiom setminusE1: !x:set.!y:set.!z:set.z iIn setminus x y -> z iIn x claim !x:set.!y:set.Subq (setminus x y) x