(a) If a trustee uses any part of the principal in the continuance of a business of which the settlor was a sole proprietor or a partner, the net profits of the business, computed in accordance with generally accepted accounting principles for a comparable business, are income. If a loss results in any fiscal or calendar year, the loss falls on principal and shall not be carried into any other fiscal or calendar year for purposes of calculating net income.
(b) Generally accepted accounting principles shall be used to determine income from an agricultural or farming operation, including the raising of animals or the operation of a nursery.