pool range utilization

[Simon Hobson]

I wrote:

> Because it's an artefact of the implementation that "never used before" addresses get allocated in a specific order - which happens in the current implementation to be "top down". That isn't documented, and it's not guaranteed to change.

Oops, as has been pointed out to me off-list, that should say "not guaranteed to not change".
Ie, it's simply an artefact of the way the addresses are hashed internally, and it could change at any time without any notice (since it's not a change in any documented feature, it might not even be in the release notes).