Definition upd_gfield_reptype t gf (v: reptype t) (v0: reptype (gfield_type t gf)) : reptype t :=
  fold_reptype
  (match t, gf return (REPTYPE t -> reptype (gfield_type t gf) -> REPTYPE t)
  with
  | Tarray t0 n a, ArraySubsc i => upd_Znth i
(*zl_concat (zl_concat (zl_sublist 0 i v) (zl_singleton i v0)) (zl_sublist (i + 1) n v) *)
  | Tstruct id _, StructField i =>
      fun v v0 => upd_compact_prod _ v (i, field_type i (co_members (get_co id))) v0 member_dec
  | Tunion id _, UnionField i =>
      fun v v0 => upd_compact_sum _ v (i, field_type i (co_members (get_co id))) v0 member_dec
  | _, _ => fun v _ => v
  end (unfold_reptype v) v0).Fixpoint upd_reptype (t: type) (gfs: list gfield) (v: reptype t) (v0: reptype (nested_field_type t gfs)): reptype t :=
  match gfs as gfs'
    return reptype (match gfs' with
                    | nil => t
                    | gf :: gfs0 => gfield_type (nested_field_type t gfs0) gf
                    end) -> reptype t
  with
  | nil => fun v0 => v0
  | gf :: gfs0 => fun v0 => upd_reptype t gfs0 v (upd_gfield_reptype _ gf (proj_reptype t gfs0 v) v0)
  end (eq_rect_r reptype v0 (eq_sym (nested_field_type_ind t gfs))).

Lemma Znth_sub_0_r: forall A {d: Inhabitant A} i l, Znth (i - 0) l = Znth i l.