Ltac solve_ptr_derives :=
  repeat rewrite isptr_offset_val;
  apply derives_refl.

Lemma field_at_Tunion: forall sh t gfs id a v1 v2 p, nested_field_type t gfs = Tunion id a -> JMeq v1 v2 -> field_at sh t gfs v1 p = nested_ufieldlist_at sh t gfs (co_members (get_co id)) v2 p.