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

Lemma wordToN_NToWord_2: forall sz n, (n < Npow2 sz)%N -> wordToN (NToWord sz n) = n.