const Union : set set const Repl : set (set set) set term famunion = \x:set.\f:set set.Union (Repl x f) const In : set set prop term iIn = In infix iIn 2000 2000 axiom ReplI: !x:set.!f:set set.!y:set.y iIn x -> f y iIn Repl x f axiom UnionI: !x:set.!y:set.!z:set.y iIn z -> z iIn x -> y iIn Union x claim !x:set.!f:set set.!y:set.!z:set.y iIn x -> z iIn f y -> z iIn famunion x f