ion and Specification in Program Development (MIT Press, Cambridge, 1986). A shorter overview of CLU appears in the article Liskov, B., et al., "Abstraction Mechanisms in CLU" (Comm. ACM, volume 20, number 8 (Aug. 1977), pages 564-576). Appendix IV summarizes the changes made to Argus that are not upward compatible with CLU. An overview and rationale for Argus is presented in Liskov, B. and Scheifler, R., "Guardians and Actions: Linguistic Support for Robust, Distributed Programs" (ACM Transactions on Programming Languages and Systems, volume 5, number 3 (July 1983), pages 381-404). The Preliminary Argus Reference Manual appeared as Programming Methodology Group Memo 39 in October 1983. Since that time several new features have been added to the language; the most significant of these are closures (see Section 9.8), a fork statement (see Section 10.4), equate modules (see Section 12.4), and a more flexible instantiation mechanism (see Section 12.6). An earlier version of this documen..