Excellent article, Frédéric. I have recommended this to people in the past.

One thing I don't quite understand about the left and right bounds. Suppose I wanted to add a new item to your hierarchy, let's say "airship". Then the left bound for airship would be 25 and the right bound 26. But 25 is already the right bound for "air" and 26 the right bound for "all". Would I need to renumber, or is the answer to leave gaps in the sequence when doing the initial numbering?

Thanks

John