If and so far as, in a case where assets have been merged or divided or have changed their nature or rights or interests in or over assets have been created or extinguished, the value of an asset is derived from any other asset in the same ownership, an appropriate proportion of the sums allowable as a deduction in the computation of a gain in respect of the other asset under paragraphs (a) and (b) of section 38(1) shall, both for the purpose of the computation of a gain accruing on the disposal of the first-mentioned asset and, if the other asset remains in existence, on a disposal of that other asset, be attributed to the first-mentioned asset.