In term of the comprehensive budget, the revenue to be generated in any budget year will equal the operating budget plus a capital-project component. The latter equals capital items purchased without debt plus the debt-service requirements (interest and repayment of principal) on borrowing for capital items purchased in prior years. Those debt costs would ideally approximate a depreciation charge for capital assets acquired in the past; serial bonds (bonds in a single project issue that are to be paid off at various dated through the life of the project) can provide a rough approximation of that cost distribution.