(a) The ad valorem tax pledged for payment of bonds shall constitute a special fund pledged as security for the payment of such indebtedness.
(b) The ad valorem tax shall never be extended for any other purpose, nor collected for any greater length of time than necessary to retire the bonded indebtedness.
(c) Upon retirement on the bonded indebtedness, any surplus tax collections which may have accumulated shall be transferred to the general fund of the issuer.
(d) The collection of ad valorem taxes, or a portion thereof, may be suspended by the issuer when not required for the payment of the bonds, subject to any covenants with the owners of the bonds.