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_compatible0_shrink: forall t_root gfsB gfsA a, field_compatible0 t_root (gfsB ++ gfsA) a -> field_compatible0 t_root gfsA a.