(a) Where a person other than the grantor of a trust has a power exercisable solely by himself to vest the corpus or the income of any portion of a testamentary or inter vivos trust in himself, he is treated under section 678(a) as the owner of that portion, except as provided in section 678(b) (involving taxation of the grantor) and section 678(c) (involving and obligation of support). The holder of such a power also is treated as an owner of the trust even though he has partially released or otherwise modified the power so that he can no longer vest the corpus or income in himself, if he has retained such control of the trust as would, if retained by a grantor, subject the grantor to treatment as the owner under sections 671 to 677, inclusive. See section 671 and §§ 1.671-2 and 1.671-3 for rules for treatment of items of income, deduction, and credit where a person is treated as the owner of all or only a portion of a trust.

(b) Section 678(a) treats a person as an owner of a trust if he has a power exercisable solely by himself to apply the income or corpus for the satisfaction of his legal obligations, other than an obligation to support a dependent (see § 1.678(c)-1 subject to the limitation of section 678(b). Section 678 does not apply if the power is not exercisable solely by himself. However, see § 1.662(a)-4 for principles applicable to income of a trust which, pursuant to the terms of the trust instrument, is used to satisfy the obligations of a person other than the grantor.