(* Restrict the code putting only codes of element in the frequency list *)
Definition restrict_code (m : list A) (c : code A) : 
  code A :=
  map (fun x => (fst x, find_code eqA_dec (fst x) c))
    (frequency_list eqA_dec m).

Theorem restrict_code_unique_key : forall c : code A, unique_key (restrict_code m c).