Hint Resolve  incl_tl incl_refl incl_app_inv_l incl_app_inv_r: coqlib.

(* Specifications: *)
Lemma incl_same_head:
forall (A: Type) (x: A) (l1 l2: list A),
incl l1 l2 -> incl (x::l1) (x::l2).