If a participant dies before separation or retirement from the Agency and is survived by a spouse or by a former spouse qualifying for a survivor annuity under section
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(b) of this title, such surviving spouse shall be entitled to an annuity equal to 55 percent of the annuity computed in accordance with paragraphs (2) and (3) of this subsection and section
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(a) of this title, and any such surviving former spouse shall be entitled to an annuity computed in accordance with section
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(b) of this title and paragraph (2) of this subsection as if the participant died after being entitled to an annuity under this subchapter. The annuity of such surviving spouse or former spouse shall commence on the day after the participant dies and shall terminate on the last day of the month before the death or remarriage before attaining age 55 of the surviving spouse or former spouse (subject to the payment and restoration provisions of sections
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(b)(3)(C),
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(h), and
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(b)(3) of this title).