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: forall sz n, exists k, wordToN (NToWord sz n) = (n - k * Npow2 sz)%N /\ (k * Npow2 sz <= n)%N.