; or (B) any portion of a foreign trust which is allocable, under such rules as the Secretary of the Treasury may prescribe, to one or more beneficiaries who are citizens or residents of the United States. (2) Grantor trust The net worth of any grantor trust shall be taken into account by the grantor in determining the tax imposed by this section. (d) Exclusion of principal residence and its indebtedness The value of any principal residence (within the meaning section 121 of such Code), and any acquisition indebtedness (as defined in section 163(h)(3)(B) of such Code) with respect thereto, shall not be taken into account under subsection (a).