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

Lemma wmsb_split1_sext: forall sz (w: word (sz + 1)), wmsb w false = wmsb (split1 _ 1 w) false -> exists sw, sext sw 1 = w.