Section 674(d) provides an additional exception to the general rule of section 674(a) for a power to distribute, apportion, or accumulate income to or for a beneficiary or beneficiaries or to, for, or within a class of beneficiaries, whether or not the conditions of section 674(b) (6) or (7) are satisfied, if the power is solely exercisable (without the approval or consent of any other person) by a trustee or trustees none of whom is the grantor or spouse living with the grantor, and if the power is limited by a reasonably definite external standard set forth in the trust instrument (see paragraph (b)(5) of § 1.674(b)-1 with respect to what constitutes a reasonably definite standard). See, however, the limitations set forth in § 1.674(d)-2.