[wp-hackers] Restructuring Category Hierarchy

[Bertrand]

> In the Nested Set, following the example [1] of Adding New Nodes,  when the
> server *trips* while shifting the other nodes (e.g. adding 2 to the lft and
> rgt columns), it would seem any subsequent query of the tree is going to be
> inaccurate, and any additions or deletions will not 'fix' the problem.
> There is no recovering.

why not manually back up the transaction in another table?
on success, remove the backup content
on failure, restore the content
if all else fails, presence of backup content on the next page load 
indicates that something failed, ask user to restore?