DEFINITION pr0_subst1_back() TYPE = ∀u2:T.∀t1:T.∀t2:T.∀i:nat.(subst1 i u2 t1 t2)→∀u1:T.(pr0 u1 u2)→(ex2 T λt:T.subst1 i u1 t1 t λt:T.pr0 t t2) BODY =Show proof