const In : set set prop term iIn = In infix iIn 2000 2000 const Empty : set const Inj1 : set set axiom Inj1I1: !x:set.Empty iIn Inj1 x const Inj0 : set set lemma !x:set.!y:set.Inj0 x = Inj1 y -> ~ Empty iIn Inj1 y claim !x:set.!y:set.Inj0 x != Inj1 y