const In : set set prop term iIn = In infix iIn 2000 2000 const ordsucc : set set axiom ordsuccI2: !x:set.x iIn ordsucc x lemma !x:set.!y:set.ordsucc x = ordsucc y -> x iIn ordsucc y -> x = y claim !x:set.!y:set.ordsucc x = ordsucc y -> x = y