Almost all of the tools developed to assist in reusing software components use one or more of the following three approaches: interactive browsers, automated retrievals based on informal specifications, and automated retrievals based on formal specifications. Browsers are easy to implement and they are provided as backup methods by many systems, including CAPS, but they rely on the user's knowledge of the structure of the library and can require examination of the entire library in the worst case. Automated retrieval facilities based on informal specifications are also provided by many systems. The most popular variants are keyword searches (CAPS, the Operation Support System OSS), multi-attribute queries based on faceted classification (CAPS, DRACO, RAPID, OSS, the Reusable Software Library, the Common Ada Missile Packages project CAMP), and natural language searches (Reusable Software Library). CAPS also supports retrievals based on formal specifications. Formal specifications can support more accurate retrievals than informal specifications, although retrievals can be potentially time consuming. The CAPS system alleviates this problem via a layered set of increasingly refined filters, so that the more time consuming methods are applied only to relatively small sets of components.