last stable version at
   [1]https://lists.cpunks.org/pipermail/cypherpunks/2022-July/102291.html
   next step might be to change flushing so the index can be accumulated
   during writes or mini flushes, so as to be able to limit the total size
   of an index per flush if needed, without issue.
   probably important to optimize away the approach of reiterating all
   leaves every flush. would greatly limit write speed on older stores,
   and be much harder to comprehend when it is old.

References

   1. https://lists.cpunks.org/pipermail/cypherpunks/2022-July/102291.html