726.4.5. An individual may deduct, in computing his taxable income for a taxation year subsequent to his taxation year 1987, the amount by which the amounts he could have deducted for the year, but for section 157.4.3, in computing his income, pursuant to sections 157.4 to 157.4.2, exceed any amount deducted under this section in computing his taxable income for a previous taxation year.