Rachel paid $1,000 for supplies in 2011. In 2012, the vendor finds a $200 mistake on the invoice and refunds the overpayment to Rachel. Which of the following doctrines or concepts is the least helpful in determining how the 2012 transaction should be reported for tax purposes?

Accounting Period.

Tax Benefit Rule.

Claim of Right.

Assignment of Income.

All-Inclusive Income.