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 setsum : set set set const Sigma : set (set set) set axiom pair_Sigma_E1: !x:set.!f:set set.!y:set.!z:set.setsum y z iIn Sigma x f -> z iIn f y const ap : set set set var x:set var f:set set var y:set var z:set hyp z iIn ap (Sigma x f) y claim setsum y z iIn Sigma x f -> z iIn f y