Variable fpad : Blist -> Blist.
Hypothesis fpad_eq: forall v l, l = Vector.to_list v ->
fpad l = Vector.to_list (fpad_v v).

Lemma app_fpad_eq v l (L:l = Vector.to_list v):
HMAC_List.app_fpad fpad l = Vector.to_list (app_fpad fpad_v v).