A taxpayer who makes an election with respect to prepaid dues income, and who includes in gross income for any taxable year to which the election applies an additional amount computed under paragraph (1), shall be permitted to deduct, for such taxable year and for each of the 4 succeeding taxable years, an amount equal to one-fifth of such additional amount, but only to the extent that such additional amount was also included in the taxpayer’s gross income during any of the 3 taxable years preceding the first taxable year to which such election applies.