38. A plan may allow an employee to elect, before the commencement of payment of the deferred annuity prescribed in section 31, to receive wholly or partly in lieu of such annuity(a) a deferred annuity reduced or increased by reason of early or deferred retirement, by provision of payment of benefits after his death or by reason of variations of such provision;
(b) a payment or series of payments by reason of a mental or physical disability.