Except as otherwise provided in the plan, no part of the corpus or income of the fund shall be used for, or diverted to, purposes other than for the exclusive benefit of members and beneficiaries entitled to benefits under the plan and paying plan expenses not otherwise paid by the city, before the satisfaction of all liabilities with respect to such members and beneficiaries. No person shall have any interest in or right to any part of the earnings of the fund, or any right in, or to, any part of the assets held under the plan, except as and to the extent expressly provided in the plan.