 the Trustee shall be under no obligation to take any action to enforce any of the provisions contained in any of the securities deposited in the Trust Fund or toward the execution or enforcement of the trust hereby created which, in its opinion, shall be likely to involve it in expense or liability, unless the Corporation or one or more of the holders of Prudence-Bonds issued hereunder shall, as often as required by the Trustee, furnish indemnity satisfactory to the Trustee against such expense or liability