"Some states ... attempt to tax nonresidents on this income on the basis that it was earned, or had its source, in the first state. Retirement income has always been a key target of this type of state taxation. Under the federal 'Source Tax Law,' however, retirement income meeting certain conditions will be taxable only by the recipient's state of residence at the time of payment, regardless of its 'source'.... The Source Tax Law ... covers retirement income paid from tax-favored vehicles such as tax-qualified retirement plans and IRAs. It also protects income from nonqualified deferred compensation arrangements if the income either is paid from a certain type of plan or in substantially equal periodic payments over life or life expectancy or a period of at least ten years."