Unless prohibited by the unitrust's governing instrument or specifically addressed by AS 13.38.350 or 13.38.420, the trustee of a unitrust may treat gains from the sale of capital assets of the unitrust as part of a distribution of principal to a beneficiary, and, if the trustee treats those gains as part of a distribution of principal to a beneficiary, the trustee shall treat those gains consistently on the unitrust's books, records, and tax returns as part of a distribution to a beneficiary.