Fixpoint splitAndPad_aux (m:list Blist) (M: Forall (fun x => length x = 512%nat) m):
           list (Bvector b) :=
(match m as l return (Forall (fun x => length x = 512%nat) l) -> list (Bvector b) with
  nil => fun F => nil
|  cons h t => fun F => cons (of_list_length h (Forall_inv F))
                    (splitAndPad_aux t (Forall_tl _ _ _ _ F))
 end) M.

Lemma splitAndPad_aux_nil m M: m=nil -> splitAndPad_aux m M= nil.