-/* INVARIANTS, in this order: bottom/top consistent, reasonable size,
- topBound consistent, space pointer, space accessible to us */
+/* INVARIANTS, in this order: reasonable size,
+ topBound consistent, space pointer, space accessible to us.
+
+ NB. This is safe to use only (a) on a spark pool owned by the
+ current thread, or (b) when there's only one thread running, or no
+ stealing going on (e.g. during GC).
+*/