Definition field_address t gfs p :=
  if (field_compatible_dec t gfs p)
  then offset_val (nested_field_offset t gfs) p
  else Vundef.Definition field_address0 t gfs p :=
  if (field_compatible0_dec t gfs p)
  then offset_val (nested_field_offset t gfs) p
  else Vundef.

Lemma field_address_app: forall t_root gfsA gfsB a, field_address t_root (gfsB ++ gfsA) a = field_address (nested_field_type t_root gfsA) gfsB (field_address t_root gfsA a).