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_compatible_range:
forall i lo hi t gfs p,
lo <= i < hi ->
field_compatible0 t (ArraySubsc lo :: gfs) p ->
field_compatible0 t (ArraySubsc hi :: gfs) p ->
field_compatible t (ArraySubsc i :: gfs) p.