A building contractor's bond, with professional surety, promises to see that all laborers and materialmen assisting upon a certain construction job are fully paid. With moneys received from work upon this building, the contractor pays a certain sum to a materialman without applying it to any particular debt. The contractor owes the materialman upon two separate debts: one for materials furnished upon this very job, and covered by this surety bond; and a pre-existing debt, in no way connected with the present contract. Is the surety able to insist that the materialman use this payment to discharge the debt on Which he is surety, or may the contractor apply it to the former debt, and later hold the surety for materials furnished on this building?