"restricted membership scheme"
means a medical scheme, the rules of which restrict the eligibility for membership by reference to—
| a) | employment or former employment or both employment or former employment in a profession, trade, industry or calling; |
| b) | employment or former employment or both employment or former employment by a particular employer, or by an employer included in a particular class of employers; |
| c) | membership or former membership or both membership or former membership of a particular profession, professional association or union; or |
| d) | any other prescribed matter. |