Fixpoint hlist_app ll lr (h : hlist ll) : hlist lr -> hlist (ll ++ lr) :=
    match h in hlist ll return hlist lr -> hlist (ll ++ lr) with
    | Hnil => fun x => x
    | Hcons _ _ hd tl => fun r => Hcons hd (hlist_app tl r)
    end.

(* Specifications: *)
Lemma hlist_app_nil_r : forall ls (h : hlist ls), hlist_app h Hnil = match eq_sym (app_nil_r_trans ls) in _ = t return hlist t with | eq_refl => h end.