[GNC] Suggestion: add "balance" variable for scheduled transactions

[jeanl]

GC people!
The scheduled transaction editor has one special variable "i" which you can
use in your credit/debit formula, and counts how many times the transaction
has posted.
I'm suggesting to add a variable "balance" that would hold the value of the
account (see below for which) at the time the transaction posts. This
balance could then be used to compute interest, and other amounts that
depend on the current balance, within the scheduled transaction.

The idea is to allow *simple* interest computations for basic fixed interest
mortgage loans that compound monthly (as is typically the case in the US,
but definitely not in the rest of the world).
Here's a typical example: You have a 4% interest rate on a mortgage with a
monthly payment of say $1000 (these have to be known in advance)
Each month the schedule transaction would transfer $1000 from your checking
account (for example) and split that into balance*0.04/12 toward an expense
account "mortgage interest" and 1000-balance*0.04/12 toward reducing the
mortgage account. "balance" would hold the current principal of the loan at
that date.

This would easily handle mortgage extra payments: If you pay an extra $500
in the middle of the month, that goes toward reducing the principal
(assuming that's how your bank handles it, but that's typical in the US) and
at the end of the month, the interests computed by the scheduled
transactions would be computed accurately according to the new loan balance.

Again, all this works only if your interests are compounded monthly and not
daily.
The account the "balance" refers to would be automatically detected based on
the nature of the accounts present in the splits (i.e., it would ignore all
accounts except for the liability account, assuming there's only one).

I understand this is far from perfect, and only applies to a subset of the
users, but for these I believe it could be quite useful. It happens to be
fairly easy to implement, which is why I'm suggesting it.

I'm welcoming any feedback on this idea.
Jean



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