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 binunion : set set set axiom binunionE: !x:set.!y:set.!z:set.z iIn binunion x y -> z iIn x | z iIn y claim !x:set.!y:set.!z:set.Subq x z -> Subq y z -> !w:set.w iIn binunion x y -> w iIn z