N.H. Rev. Stat. Ann. § 162-G:12-a

The governmental unit, by resolution of its governing body, may covenant with holders of its bonds, issued under this chapter, in such manner and to such extent as the governing body deems necessary, that the governmental unit will act in such a way as to preserve, or refrain from acting in such a way as to defeat, any exemption from federal income tax which may be applicable with respect to interest on such bonds at the time when they are issued.