Definition value_fits: forall t, reptype t -> Prop :=
  type_func (fun t => reptype t -> Prop)
    (fun t v =>
       if type_is_volatile t then True else tc_val' t (repinject t v))
    (fun t n a P v => Zlength (unfold_reptype v) =  Z.max 0 n /\ Forall P (unfold_reptype v))
    (fun id a P v => struct_value_fits_aux (co_members (get_co id)) (co_members (get_co id)) P (unfold_reptype v))
    (fun id a P v => union_value_fits_aux (co_members (get_co id)) (co_members (get_co id)) P (unfold_reptype v)).

Lemma data_at_rec_share_join: forall sh1 sh2 sh t v b ofs, sepalg.join sh1 sh2 sh -> data_at_rec sh1 t v (Vptr b ofs) * data_at_rec sh2 t v (Vptr b ofs) = data_at_rec sh t v (Vptr b ofs).