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_field_compatible0:
forall (t : type) (gfs : list gfield) (p : val),
field_compatible t gfs p -> field_compatible0 t gfs p.