Summing & Grouping in a Hierarchical Structure

I posted to this list some time ago about working with a hierarchical category structure.   I had great difficulty with my problem and gave up for a time.  I recently returned to it and resolved a big part of it.  I have one step left to go, but at least I have solved this part.

Here is the original thread (or one of them):
http://www.postgresql.org/message-id/CAJ-7yonw4_qDCp-ZNYwEkR2jdLKeL8nfGc+-TLLSW=Rmo1VkbA@mail.gmail.com
Here is my recent blog post about how I managed to show my expenses summed and grouped by a mid-level category:
http://dcparris.net/2013/02/13/hierarchical-categories-rdbms/


Specifically, I wanted to sum and group expenses according to categories, not just at the bottom tier, but at higher tiers, so as to show more summarized information.  A CEO primarily wants to know the sum total for all the business units, yet have the ability to drill down to more detailed levels if something is unusually high or low.  In my case, I could see the details, but not the summary.  Well now I can summarize by what I refer to as the 2nd-level categories.