(a) An obligation qualifies as a Type I security if it is secured by an escrow fund consisting of obligations of the United States or general obligations of a State or a political subdivision, and the escrowed obligations produce interest earnings sufficient for the full and timely payment of interest on, and principal of, the obligation.

(b) If the interest earnings from the escrowed Type I securities alone are not sufficient to guarantee the full repayment of an obligation, a promise of a State or a political subdivision possessing general powers of taxation to maintain a reserve fund for the timely payment of interest on, and principal of, the obligation may further support a guarantee of the full repayment of an obligation.

(c) An obligation issued to refund an indirect general obligation may be supported in a number of ways that, in combination, are sufficient at all times to support the obligation with the full faith and credit of the United States or a State or a political subdivision possessing general powers of taxation. During the period following its issuance, the proceeds of the refunding obligation may be invested in U.S. obligations or municipal general obligations that will produce sufficient interest income for payment of principal and interest. Upon the retirement of the outstanding indirect general obligation bonds, the same indirect commitment, such as a lease agreement or a reserve fund, that supported the prior issue, may support the refunding obligation.