For example, operations and maintenance funds generally cannot be used to purchase supplies after 30 September of the year they are appropriated within with several exceptions – 1) the severable services exemption under 10 USC 2410 and Office of Management and Budget (OMB) Circular A-34, Instructions on Budget Execution, 2) Authorized stockage level exceptions and 3) long lead time exception. (see https://web.archive.org/web/20041027004108/https://www.safaq.hq.af.mil/contracting/affars/fiscal-law/bona-fide-need.doc ) The Government Accounting Office Principles of Federal Appropriations Law (otherwise known as the GAO Redbook at http://www.gao.gov/legal.htm ) has a detailed discussion of these fiscal law rules which directly impact on the ability of a Federal agency to contract with the private sector.