const Eps_i : (set prop) set const In : set set prop term iIn = In infix iIn 2000 2000 term inv = \x:set.\f:set set.\y:set.Eps_i \z:set.z iIn x & f z = y axiom Eps_i_ax: !p:set prop.!x:set.p x -> p (Eps_i p) claim !x:set.!y:set.!f:set set.(!z:set.z iIn y -> ?w:set.w iIn x & f w = z) -> !z:set.z iIn y -> inv x f z iIn x & f (inv x f z) = z