of the regular tax liability (as defined in section 26(b)) plus the tax imposed by section 55, over (B) the sum of the credits allowable under this part (other than subpart C and this subpart). (2) Carryover of unused credit If the credit allowable under subsection (a) exceeds the limitation imposed by paragraph (1) for such taxable year, such excess shall be carried to the succeeding taxable year and added to the credit allowable under subsection (a) for such taxable year (determined before the application of paragraph (1) for such succeeding taxable year). (d) School infrastructure bond (1) In general For purposes of this section, the term school infrastructure bond means any bond issued as part of an issue if— (A) 100 percent of the available project proceeds of such issue are to be used for the purposes described in section 101(c)(1) of the School Building Improvement Act of 2017 , (B) the interest on such obligation would (but for this section) be excludable from gross income under section 103, (C) the issue meets the requirements of paragraph (3), and (D) the issuer designates such bond for purposes of this section. (2) Applicable rules For purposes of applying paragraph (1)— (A) for purposes of section 149(b), a school infrastructure bond shall not be treated as federally guaranteed by reason of the credit allowed under subsection (a) or section 6431, (B) for purposes of section 148, the yield on a school infrastructure bond shall be determined without regard to the credit allowed under subsection (a), and (C) a bond shall not be treated as a school infrastructure bond if the issue price has more than a de minimis amount (determined under rules similar to the rules of section 1273(a)(3)) of premium over the stated principal amount of the bond. (3) 6-year expenditure period (A) In general An issue shall be treated as meeting the requirements of this paragraph if, as of the date of issuance, the issuer reasonably expects 100 percent of the available project proceeds to be spent for purposes described in