¹[The following contracts cannot be specifically enforced, namely:--

   (a)  where a party to the contract has obtained substituted performance of contract in accordance with the provisions of section 20;

   (b)  a contract, the performance of which involves the performance of a continuous duty which the court cannot supervise;

   (c)  a contract which is so dependent on the personal qualifications of the parties that the court cannot enforce specific performance of its material terms; and

   (d)  a contract which is in its nature determinable.]