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_clarify:
forall t path c,
is_pointer_or_null (field_address t path c) ->
field_address t path c = offset_val (nested_field_offset t path) c.