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_change_composite: forall {cs_from cs_to} {CCE: change_composite_env cs_from cs_to} (t: type), cs_preserve_type cs_from cs_to (coeq _ _) t = true -> forall gfs p, @field_compatible cs_from t gfs p <-> @field_compatible cs_to t gfs p.