The grantor shall be treated as the owner of any portion of a trust, whether or not he is treated as such owner under section
674, whose income without the approval or consent of any adverse party is, or, in the discretion of the grantor or a nonadverse party, or both, may be—
This subsection shall not apply to a power the exercise of which can only affect the beneficial enjoyment of the income for a period commencing after the occurrence of an event such that the grantor would not be treated as the owner under section
673 if the power were a reversionary interest; but the grantor may be treated as the owner after the occurrence of the event unless the power is relinquished.