Deduction basis under Section 80MM: changed to net income by Section 80AB, leaving prior assessments governed by gross income rule. Deduction under Section 80MM was judicially held to be computed with reference to gross income, but insertion of Section 80AB changed the computation to net income. Section 80AB was made effective from 1 April 1981 and not retrospectively; therefore the gross income rule applies to assessments up to and including assessment year 1980-81, while subsequent assessments must apply the net income basis.