(** [is_tail l1 l2] holds iff [l2] is of the form [l ++ l1] for some [l]. *)
Inductive is_tail (A: Type): list A -> list A -> Prop :=
| is_tail_refl:
forall c, is_tail c c
| is_tail_cons:
forall i c1 c2, is_tail c1 c2 -> is_tail c1 (i :: c2).

Lemma is_tail_in:
forall (A: Type) (i: A) c1 c2, is_tail (i :: c1) c2 -> In i c2.