proposition: Lemma varUD_tail_rev : forall E i, tail E |= i :! -> E |= S i :! .
proof: Proof. intros; destruct E; trivial. destruct i; trivial. Qed.
imports: CoLoR(RelExtras ListExtras TermsLifting LogicUtil OptUtil) Coq(Arith Setoid Morphisms Lia)