Section Facts.
Variable A : Type.
(** *** Generic facts *)
(** Destruction *)
(** *** Head and tail *)
(************************)
(** *** Facts about [In] *)
(************************)
(** Characterization of [In] *)
Hint Resolve app_assoc_reverse : core.
(* end hide *)
(** Facts deduced from the result of a concatenation *)
(** Compatibility with other operations *)

Lemma app_inv_head:
forall l l1 l2 : list A, l ++ l1 = l ++ l2 -> l1 = l2.