687.1. For the purposes of sections 83 to 85.6, the fair market value at any time of a capital interest in a trust is deemed to be equal to the aggregate of(a) the amount that would, if this Part were read without reference to this section, be its fair market value at that time; and
(b) the aggregate of all amounts, each of which is an amount in respect of the capital interest that became payable to the taxpayer before that time and that would be described in subparagraph i.1 of paragraph n of section 257 if its subparagraph 3 were read without reference to “, that is, subject to section 257.4, equal to the amount designated by the trust to be payable to the taxpayer under section 668”.