179. (1) A taxpayer may deduct an amount paid in the year to pay the principal amount of a bond, debenture, bill, hypothecary claim, mortgage or other similar obligation, but only if they have been issued by the taxpayer after 18 June 1971 and call for the payment of interest and only to the extent that the amount so paid does not exceed:(a) where such security has been issued for an amount not less than 97% of its principal amount, and its yield, expressed in yearly percentage on the amount for which it has been issued does not exceed 4/3 of the annual rate of interest stipulated, the amount according to which the lesser of the principal amount of such security and the aggregate of amounts paid in the year or in a previous year to repay its principal amount exceeds the amount for which it has been issued; and
(b) in all other cases, the lesser of 1/2 of the amount so paid and 1/2 of the amount by which the lesser of the principal amount of the security and the aggregate of the amounts paid in the year or in any preceding taxation year in satisfaction of the principal amount thereof exceeds the amount for which it has been issued.