Thank you.  And, heck yes, more people need to be talking about how to deal with computations that are largely repeated.  That problem comes up all over the place.<p>Couchdb had an interesting approach for this that did not require inverses: they persist intermediate reduce results, so that you can re-use nodes in the tree that haven&#x27;t changed. So clever. <a href="http:&#x2F;&#x2F;horicky.blogspot.com&#x2F;2008&#x2F;10&#x2F;couchdb-implementation.html" rel="nofollow">http:&#x2F;&#x2F;horicky.blogspot.com&#x2F;2008&#x2F;10&#x2F;couchdb-implementation.h...</a>