const In : set set prop term iIn = In infix iIn 2000 2000 const setsum : set set set const Sigma : set (set set) set axiom lamI: !x:set.!f:set set.!y:set.y iIn x -> !z:set.z iIn f y -> setsum y z iIn Sigma x f const ordsucc : set set const Empty : set const If_i : prop set set set axiom pair_tuple_fun: setsum = \x:set.\y:set.Sigma (ordsucc (ordsucc Empty)) \z:set.If_i (z = Empty) x y claim !x:set.!f:set set.!y:set.y iIn x -> !z:set.z iIn f y -> Sigma (ordsucc (ordsucc Empty)) (\w:set.If_i (w = Empty) y z) iIn Sigma x \w:set.f w