50A. Special provision for cost of acquisition in case of depreciable asset.—Where the capital asset is an asset in respect of which a deduction on account of depreciation under clause (i) of sub-section (1) of section 32 has been obtained by the assessee in any previous year, the provisions of sections 48 and 49 shall apply subject to the modification that the written down value, as defined in clause (6) of section 43, of the asset, as adjusted, shall be taken as the cost of acquisition of the asset.