progress

I can now create branch x,
fork branch x y,
delete branch x and then
delete branch y.

This works great, after a whole lot of effort. Also I now have a solid validation tool for checking for duplicate space usage and memory leaks in the database. In addition, I've sped things up by fixing a bug which forced every object read to be rewritten. So the effort has been very worthwhile.

But when I create branch y z, i.e. fork a fork, I still have problems. So I'm close, but no cigar.