Depreciation adjustment: retrospective rule treats book revaluation as altering asset cost and allowable depreciation for prior uncomputed years. Explanation 6 to clause (6) of section 43 provides that where an assessee did not compute total income for prior years, (a) the asset's actual cost shall be adjusted by any book revaluation amount, (b) total depreciation shown in the books for those prior years shall be deemed to be the depreciation actually allowed under the Act for this clause, and (c) that deemed depreciation shall be adjusted to exclude the portion attributable to the asset's revaluation.