proposition: Lemma In_appr {X : Type} {x: X} {l1 l2: list X} : In x l2 -> In x (l1 ++ l2).
proof: Proof. move=> ?. apply: in_or_app. by right. Qed.
imports: List Relation_Operators Operators_Properties PeanoNat Lia ssreflect ssrbool ssrfun