(b)the 1st day of April, 1992, or to any one of the four assessment years next following that assessment year, includes] [any income derived by him in the exercise of his profession on account of any lump sum consideration for the assignment or grant of any of his interests in the copyright of any book, or of royalties or copyright fees (whether receivable in lump sum or otherwise) in respect of such book, there shall, in accordance with and subject to the provisions of this section, be allowed, in computing the total income of the assessee, a deduction from such income of an amount equal to twenty-five per cent. thereof. [Inserted by Act 21 of 1979, Section 15 (w.e.f. 1.4.1980).]