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 binunionI1: !x:set.!y:set.!z:set.z iIn x -> z iIn binunion x y claim !x:set.!y:set.Subq x (binunion x y)