The board of trustees shall hold the assets of the system in trust for the exclusive purpose of providing benefits to participants and paying reasonable expenses of administration. It shall be impossible at any time prior to the satisfaction of all liabilities to members and beneficiaries covered by the trust, by operation of the system, by termination, by power of revocation or amendment, by the happening of any contingency, by collateral arrangement or by other means, for any part of the corpus or income of the trust, or any funds contributed thereto, to inure to the benefit of any employer or otherwise be used for or diverted to purposes other than providing benefits to members and beneficiaries and defraying reasonable expenses of administering the system.