const In : set set prop term iIn = In infix iIn 2000 2000 term nIn = \x:set.\y:set.~ x iIn y const Sing : set set axiom SingE: !x:set.!y:set.y iIn Sing x -> y = x const ordsucc : set set const Empty : set axiom neq_1_2: ordsucc Empty != ordsucc (ordsucc Empty) hyp Sing (ordsucc (ordsucc Empty)) iIn Sing (Sing (ordsucc Empty)) claim ~ ordsucc Empty iIn Sing (ordsucc (ordsucc Empty))