(a) For the purposes of this section, "undistributed income" means net income received before the date on which an income interest ends. The term does not include an item of income or expense that is due or accrued or net income that has been added or is required to be added to principal under the terms of the trust.
(b) Except as provided in subsection (c), on the date when a mandatory income interest ends, the fiduciary shall pay to a mandatory income beneficiary who survives that date, or the estate of a deceased mandatory income beneficiary whose death causes the interest to end, the mandatory income beneficiary's share of the undistributed income that is not disposed of under the terms of the trust.
(c) If, immediately before the income interest ends, the mandatory income beneficiary has an unqualified power to revoke more than five percent of the trust, then the undistributed income from the portion of the trust that may be revoked shall be added to principal.
(d) When a fiduciary's obligation to pay a fixed annuity or a fixed fraction of the value of the trust's assets ends, the fiduciary shall prorate the final payment.