Transitive commodity values / Pegging

[Lover O'Bean]

I have defined three currencies which should all be equal to the current IRS Standard Business mileage rate[1], call them IRSB.T, IRSB.R, IRSB.F (tax-deductible, reimbursable, and "fixed" for when I borrow a vehicle from a friend or relative).  I would like to "peg" IRSB.R and IRSB.F to the official rate so that I only update[2] one.  What is the best practice?

1) Always update everything:

2023-01-01 price IRSB.T 0.655 USD

2023-01-01 price IRSB.R 0.655 USD

2023-01-01 price IRSB.F 0.655 USD

;

2024-01-01 price IRSB.T 0.67 USD

2024-01-01 price IRSB.R 0.67 USD

2024-01-01 price IRSB.F 0.67 USD

2) Update one and set the others equal every time

2023-01-01 price IRSB.T 0.655 USD

2023-01-01 price IRSB.R 1 IRSB.T

2023-01-01 price IRSB.F 1 IRSB.T

;

2024-01-01 price IRSB.T 0.67 USD

2024-01-01 price IRSB.R 1 IRSB.T

2024-01-01 price IRSB.F 1 IRSB.T

3) Set the "peg" initially and then only update one

2023-01-01 price IRSB.T 0.655 USD

2023-01-01 price IRSB.R 1 IRSB.T

2023-01-01 price IRSB.F 1 IRSB.T

2024-01-01 price IRSB.T 0.67 USD

I assume that 3) will not work and that IRS.R and IRS.F would remain stuck at the old rate?

I apologize that I don't have enough transactions yet to test this thoroughly.

[1] The Beancount documentation talks about opening currencies when they were first created.  To spare others the agony, the 1964 1040 instructions seem to be the first to clearly reference a standard mileage rate: there was a major tax reform in 1964, but I didn't exactly see a reference to a standard mileage rate as part of that bill.

[2] It seems that rate changes are usually announced through "IRS NEWSWIRE" service; email alerts for this service can be obtained through subscribing here:

https://service.govdelivery.com/service/multi_subscribe.html?code=USIRS

[Martin Blais]

Beancount doesn't automatically compute implied prices based on transitive prices.

All it does is impute the inverse price, and project the prices forward (the last valid price--in time--is the one that's used).

You'll have to just update all of them.

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[Lover O'Bean]

That is fine; thanks for the clarification!