Hint Resolve is_tail_refl is_tail_cons is_tail_in is_tail_cons_left is_tail_nil: coqlib.

(* Specifications: *)
Lemma is_tail_incl:
forall (A: Type) (l1 l2: list A), is_tail l1 l2 -> incl l1 l2.
Lemma is_tail_trans:
forall (A: Type) (l1 l2: list A),
is_tail l1 l2 -> forall (l3: list A), is_tail l2 l3 -> is_tail l1 l3.