intros l; elim l; simpl in |- *; auto.
Qed.
Hint Resolve is_prefix_refl : core.
(* A code is unique_prefix, if there is no encoding that is prefix of another *)
Definition unique_prefix (l : code) :=
(forall (a1 a2 : A) (lb1 lb2 : list bool),
In (a1, lb1) l -> In (a2, lb2) l -> is_prefix lb1 lb2 -> a1 = a2) /\
unique_key l.

(* Specifications: *)
(* The empty code is unique prefix *)
Theorem unique_prefix_nil : unique_prefix nil.
Proof using.