A single child is just another condition to watch out for during B-tree
merge, since a single-child obviously can't have a sibling.
This is a good safety to have, but I was surprised this can happen. But
it turns out to be quite easy since our rbyds defer the B-tree
operations until compaction. A merge down to a single child won't
propagate the merge until the parent compacts.
Christopher Haster
dd5af724fd