Definition only_blocks {S: block -> Prop} (S_dec: forall b, {S b}+{~S b}) (w: rmap) : rmap.
refine (proj1_sig (make_rmap (fun loc => if S_dec (fst loc) then w @ loc else core (w @ loc))
_ (level w) _ (ghost_of_approx w))).

Lemma join_only_blocks:
forall {S} S_dec phi, identity (ghost_of phi) -> join (@only_blocks S S_dec phi)
(only_blocks (not_dec S_dec) phi) phi.