Coercion seq_In T : seq T -> set T := fun s x => List.In x s.Coercion list_In T : list T -> set T := fun s x => List.In x s.

Lemma incl_hd_same {A : Type} (a : A) (l1 l2 : seq A) : incl l1 l2 -> incl (a :: l1) (a :: l2).