Proof using.
intros. subst. in_crush.
Qed.
Hint Resolve app_cons_in_rest : struct_util.

(* Specifications: *)
Lemma in_rest_app_cons:
forall (l xs ys : list A) a b,
l = xs ++ a :: ys ->
In b l ->
a <> b ->
In b (xs ++ ys).