“ In the case of a bond given under a statute such as the act of February 28, 1899, there is no single obligee or creditor. The surety is charged with notice that he is entering into what is, in a very proper sense, a public obligation, and one that will be relied upon by persons who can in no manner control the conduct of the nominal obligee, and with respect to whom the latter is a mere trustee, and therefore incapable, upon general principles of equity, of bartering away, for its own benefit or convenience, the rights of the beneficiaries. ”

“ But, with respect to obligations incurred by the contractor to laborers and materialmen, at least, so far as their labor and materials are supplied in accordance with the original contract, it is obvious, we think, that a construction which would discharge the surety because of any change to which the laborers and materialmen were not parties would defeat the principal object that Congress had in view in enacting the statute. ”

“ The nominal obligee is, with respect to these third parties, a mere trustee, and the obligors, including the surety as well as the principal contractor, enter into the obligation in full view of this. ”