proposition: Lemma Exists_drop_inv : forall P n l, Exists P (drop n l) -> Exists P l.
proof: Proof. Proof using. introv E. forwards (q&K): drop_is_suffix n l. rewrite K. rewrite* Exists_app_eq. Qed.
imports: Coq.Classes.Morphisms TLC(LibTactics LibLogic LibReflect LibOperation LibProd LibOption LibNat LibInt LibWf LibMonoid LibRelation)