Statement Triggers and access to other rows in the transaction

I have tables representing a ledger system and the accounts on which it
operates.

I want to do 2 things to all transactions altering the ledger tables.

First, all of the inserts into the ledger tables should be balanced ( They
should sum to 0 ). If not I want to abort the transaction.

Second, if the transaction is balanced I want to update the account balance on
the account tables.

In other database systems that I have used when you have a statement level
trigger you get access to the inserted/updated/deleted rows from that
transaction as "virtual" tables within the trigger that you can manipulate.

I am currently using plpgsql to write my triggers. I have not been able to
figure out the "right" way to do this in PostgreSQL and would appreciate any
advice.

Thanks,

Craig