26 U.S.C. § 86 — Social security and tier 1 railroad retirement benefits
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If— (A) any portion of a lump-sum payment of social security benefits received during the taxable year is attributable to prior taxable years, and (B) the taxpayer makes an election under this subsection for the taxable year, then the amount included in gross income under this section for the taxable year by reason of the receipt of such portion shall not exceed the sum of the increases in gross income under this chapter for prior taxable years which would result solely from taking into account such portion in the taxable years to which it is attributable.
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