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