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

Lemma of_inj_pos d d' : Nat.of_int (Pos d) = Nat.of_int (Pos d') -> unorm d = unorm d'.