In the case of a taxable year to which section
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(a)(2) applies, if the credit allowable under subsection (a) exceeds the limitation imposed by section
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(a)(2) for such taxable year reduced by the sum of the credits allowable under this subpart (other than this section), such excess shall be carried to the succeeding taxable year and added to the credit allowable under subsection (a) for such succeeding taxable year.