Neb. Rev. Stat. § 46-1,124

Upon the discharge of the judgments and the surrender of the notes, warrants and bonds, and the issuance of the new bonds, the resolution approved by the electors shall constitute a contract with the holders of the bonds, and all facts therein recited shall conclusively be deemed to be true as against the district and in favor of the holders of the bonds. A recital in the bonds that they are issued in pursuance of sections 46-1,114 to 46-1,126 shall be conclusive evidence that such bonds are valid and the same shall be incontestable.