Ltac simpl_of_lu :=
  match goal with
  | |- context [ of_lu (?f ?x) ] =>
    rewrite (of_lu_eqn (f x)); simpl hd; simpl tl
  end.

Lemma to_int_inj n n' : Nat.to_hex_int n = Nat.to_hex_int n' -> n = n'.