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 var x:set var f:set set var y:set hyp !z:set.z iIn x -> !w:set.w iIn x -> f z = f w -> z = w hyp y iIn x claim inv x f (f y) iIn x & f (inv x f (f y)) = f y -> inv x f (f y) = y