Ltac wlt_ind :=
  match goal with
  | [ |- forall (n: word ?len), ?P ] =>
    refine (well_founded_ind (@wlt_wf len) (fun n => P) _)
  end.

Theorem wordToNat_minus_one: forall sz n, n <> natToWord sz 0 -> wordToNat (n ^- natToWord sz 1) = wordToNat n - 1.