(**** tactics for value_fits  ****)

Lemma value_fits_Tunion:
forall {cs: compspecs} t (v: reptype t) i a m v2 r,
t = Tunion i a ->
m = co_members (get_co i)  ->
JMeq (@unfold_reptype cs t v) v2 ->
r =union_Prop m
(fun it => value_fits (field_type (fst it) m))  v2 ->
value_fits t v = r.