In a taxable year in which an amount is included in a beneficiary's income under § 1.668(a)-1A(a), the tax on the beneficiary for such taxable year is determined only as provided in section 668 and consists of the sum of:

(a) A partial tax computed on (1) the beneficiary's taxable income reduced by (2) an amount equal to the total amounts includible in his income under § 1.668(a)-1A(a), at the rate and in the manner as if section 668 had not been enacted,

(b) A partial tax determined as provided in § 1.668(b)-1A, and

(c) In the case of a beneficiary of a trust which is not required to distribute all of its income currently, a partial tax determined as provided in § 1.669(b)-1A.