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 only_blocks_at: forall {S} S_dec phi loc,
@only_blocks S S_dec phi @ loc =
if S_dec (fst loc) then phi @ loc else core (phi @ loc).