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

Lemma existT_wminus: forall sz (w1 w2: word sz) sz' (w3 w4: word sz'), existT word _ w1 = existT word _ w3 -> existT word _ w2 = existT word _ w4 -> existT word _ (w1 ^- w2) = existT word _ (w3 ^- w4).