[development] Lineage module --- nested trees

[Moshe Weitzman]

> Thanks Earl.  I hope it is possible to come up with a generic  
> implementation that is elegant; it could significantly speed up the  
> book module.

I should note that the book.module actually supports a multiple-parent 
model. It is just the UI that prohibits it. See 
http://drupal.org/node/5901. I'd like to see us truly support this, 
instead of falling back to single parent model.