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 claim !x:set.!f:set set.!y:set.y iIn x -> !z:set.z iIn f y -> setsum y z iIn Sigma x \w:set.f w