In computing its undistributed income on hand at the end of its 1949 taxation year, the taxpayer was entitled to deduct "all capital losses sustained" by it. The taxpayer was not entitled to a deduction in respect of a decline in the value of shares. "The phrase 'capital losses sustained' or its equivalent appears in several provisions of the statute in a context from which it is apparent that, within the conceptions of accountancy underlying the Act, it means actually realized ...