| (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.] |