This works fine with how we're currently using it, makes transformation
methods easier to implement, and avoids potential bugs where a balance is
initialized with a bad mapping.
This was an early mistake, it makes data consistency mistakes too
easy, and I only used it once so far in actual code. Going to fix
this now so I can more safely build on top of this data structure.
payment-report and accrual-report query to find the last date a
series of postings had a non/zero balance. This method is a good
building block for that.
