13.   Exemption.-

(1)    Notwithstanding the provisions of section 3, supply of goods or import of goods specified in the Sixth Schedule shall, subject to such conditions as may be specified by the Federal Government, be exempt from tax under this Act:

Provided that the Federal Government may, by notification in the official Gazette, withdraw any exemption granted under the Sixth Schedule to the extent specified in the notification:

Provided further that the aforesaid power to withdraw an exemption shall not be construed to include the power to revive or to restore the exemption so withdrawn.

(4)    Where a person does not desire to avail any tax exemption, he may, after voluntary registration, opt to pay sales tax at the rate applicable to such supplies under the provisions of section 3 subject to condition that he shall not thereafter be de-registered till the expiry of two years from the date of such registration.

(5)    Notwithstanding the omission of sub-section (2), the Federal Government may, by notification in the official Gazette, rescind a notification issued by it or by the Board under sub-section (2) prior to the first day of December, 1998.