Cascading sum in tree with CTE?

Hi .

My problem resembles this:

I have the following (simplified) tables

1) create table account ( id serial, name varchar, parent_id int4
references account, primary key (id))
2) create table transaction (id serial, account_id int4 references
account, memo varchar, debet, credit, primary key(id))

So a basic ledger system, with a tree of accounts and transactions on
them. Some accounts have transactions others doesn't.

I have written a CTE which generates the tree, but I would like to
append to each line the sum of all debits and credits of this and all
sub accounts.
I think it sould be doable, but cannot bend my brain around it and my
google-fu has come out short.

Say the data was:

Account:
1, 'Expenses', null
2, 'IRS', 1
3, '7-Eleven'

Transaction:
1, 2, 'Tax Jan 10', null, 100
2, 2, 'Tax Feb 10', null, 120
3, 2, 'Tax Feb 10 correction', 10,null
4, 3, 'Sodas', 10, null
5, 3, 'Beer', 5, null

I would expect a tree like:

1, Debt, 25, 220
2, IRS, 10, 220
3, 7-eleven, 15, 0

Is there any way around a writing a stored procedure for that?

My current CTE works top down ( top > Expenses > 7-elven) and writes out "paths" to the leaves (ie "Expenses -> 7-Eleven" instead of just 7-Eleven)

Svenne