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 nested_field_type_ArraySubsc: forall t i gfs,
nested_field_type t (ArraySubsc i :: gfs) = nested_field_type t (ArraySubsc 0 :: gfs).