assets up to a specified amount. Correspondingly, the program-wide liquidity facility provides funds to the conduit to ensure the timely payment of maturing paper for reasons other than defaults (e.g., market disruptions). Since investors are exposed to defaults of the underlying assets, the NRSROs make their expected perfor- mance under various scenarios a central focus of the ratings process.   Single-Seller versus Multi-Seller Programs The other key dimension used to categorize ABC paper conduits is as either single-seller or multiseller. Single-seller conduits securitize assets purchased from a single seller (e.g., a single originator). Conversely, mul- tiseller conduits pool assets purchased from several disparate sellers and the ABC paper issued is backed by the portfolio of these assets.   Credit and Liquidity Enhancement In a multiseller partially supported ABC paper conduit, there are two lev- els of credit enhancement. The first line of defense is pool-specific credit enhancement that provides protection from the defaults on assets from a particular seller. Pool-specific credit enhancement may include overcollat- eralization, third-party credit support, or excess spread. The second line of defense is program-wide credit enhancement that provides protection after the pool-specific credit enhancement is depleted. Program-wide credit enhancement is usually supplied by a third-party in the form of an irrevocable loan facility, letter of credit, surety bond from a monoline insurance company, or cash invested in permitted securities.7 Liquidity enhancement is also structured in two levels-pool-specific or program-wide. Liquidity enhancement usually takes the one of two forms. One form of liquidity support is a loan agreement in which the liquidity facility agrees to extend loans to the conduit if maturing paper cannot be rolled over due to say, a disruption in the commercial paper market due to a financial crisis. Note that the liquidity facility is not     7"Understanding Asset-Backed Commercial Paper."     responsible for interjecting needed funds into the conduit due to defaults in the asset portfolio. The other form of liquidity support is an asset pur- chase agreement in which the liquidity facility agrees to purchase non- defaulted assets if funds are needed. Exhibit 5.10 presents a flow chart illustrating the basic structure of a partially supported, multiseller ABC paper program. Note the administra- tive agent invests no cash into the deal but instead provides a flow of ser- vices, as a result, the administrative agents connection to the conduit is represented with a dashed line.