const In : set set prop term iIn = In infix iIn 2000 2000 const Repl : set (set set) set axiom ReplEq: !x:set.!f:set set.!y:set.y iIn Repl x f <-> ?z:set.z iIn x & y = f z claim !x:set.!f:set set.!y:set.y iIn x -> f y iIn Repl x f