R.I. Gen. Laws § 45-48-12
The district may at any time deposit with the trustee, under its then existing bond resolution a sum sufficient, with amounts then on deposit, including the debt service reserve fund, to purchase direct obligations of the United States of America which are adequate to pay the entire principal amount of the bonds secured by the resolution, together with the interest to maturity, or to an applicable redemption date specified by the district to the trustee and any applicable redemption premium; or the district may deposit direct obligations of the United States of America in lieu of money for their purchase. The obligations are deemed adequate if the principal and interest payable on them are sufficient to pay the previously mentioned sums when due. Upon any deposit of money and a request by the district, the trustee shall purchase direct obligations of the United States of America. When adequate direct obligations of the United States of America are held by the trustee pursuant to this section, the bond resolution shall cease to be in effect. The obligations and their proceeds shall be held in trust for the benefit of the bondholders, and the trustee shall, on behalf of the district, call bonds for redemption on the applicable redemption date specified pursuant to this section. Any compensation or expenses of the trustee in carrying out this section shall be paid by the district, and any surplus funds held by the trustee under this section shall be remitted by the trustee to the district.
R.I. Gen. Laws § 45-48-12