671.3. For the purposes of sections 146.1 and 772.2 to 772.13, there shall be deducted in computing the business-income tax or non-business-income tax, as the case may be, paid by a trust to the government of a foreign country or political subdivision of a foreign country in respect of a particular source situated in that country for a taxation year the aggregate of all amounts each of which is an amount deemed, because of a designation under section 671 by the trust for the year, to be paid by beneficiaries under the trust as business-income tax or non-business-income tax, as the case may be, in respect of that source.