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 ordsucc : set set const Empty : set const If_i : prop set set set var x:set var y:set var z:set var w:set hyp w iIn If_i (z = Empty) x y hyp z = ordsucc Empty claim If_i (z = Empty) x y = y -> w iIn y