The Secretary shall prescribe regulations setting forth the application of this section and section
6324B in the case of an interest in a partnership, corporation, or trust which, with respect to the decedent, is an interest in a closely held business (within the meaning of paragraph (1) of section
6166
(b)). For purposes of the preceding sentence, an interest in a discretionary trust all the beneficiaries of which are qualified heirs shall be treated as a present interest.