Deduction for specified income restricted to a single limited allowance, excluding other section 57 deductions. An amendment to section 57 provides that income referred to in clause (viii) of sub section (2) of section 56 shall be allowed a deduction equal to fifty per cent of such income and that no deduction shall be allowed under any other clause of section 57 in respect of that income, effective from the first day of April, 2010.