1121.5. For the purposes of paragraph a of section 1121.1 and section 1121.4, the amount by which the aggregate of all amounts each of which is an amount designated by a trust for a particular taxation year under section 1121.1 exceeds the amount deductible under section 1121.1 in computing its income for the particular year is deemed to be an amount designated by the trust under section 1121.1 for its taxation year following the particular year.