const Inj1 : set set const Empty : set axiom Inj1NE1: !x:set.Inj1 x != Empty const In : set set prop term iIn = In infix iIn 2000 2000 const Inj0 : set set axiom Inj0E: !x:set.!y:set.y iIn Inj0 x -> ?z:set.z iIn x & y = Inj1 z var x:set var y:set hyp Inj0 x = Inj1 y hyp Empty iIn Inj1 y claim ~ Empty iIn Inj0 x