of Micro-Planner which were pragmatically useful for the computers of the era because they saved space and time: Backtracking control structure Unique Name Assumption by which different names are assumed to refer to distinct entities, e.g., Peking and Beijing are assumed to be different. Reification of Failure. The way that Planner established that something was provable was to successfully attempt it as a goal and the way that it establish that something was unprovable was to attempt it as a goal and explicitly fail. Of course the other possibility is that the attempt to prove the goal runs forever and never returns any value. Planner also had a construct which succeeded if failed, which gave rise to the “Negation as Failure” terminology in Planner. Use of the Unique Name Assumption and Negation as Failure became more questionable when attention turned to Open Systems [Hewitt and de Jong 1983, Hewitt 1985, Hewitt and Inman 1991]. The following capabilities of Micro-Planner were omitted from Prolog: Pattern-directed invocation of procedural plans from assertions (i.e., forward chaining) Logical negation, e.g., . Prolog did not include negation in part because it raises implementation issues. Consider for example if negation were included in the following Prolog program: The above program would be unable to prove even though it follows by the rules of mathematical logic. This is an illustration of the fact that Prolog (like Planner) is intended to be a programming language and so does not (by itself) prove many of the logical consequences that follow from a declarative reading of its programs. The work on Prolog was valuable in that it was much simpler than Planner. However, as the need arose for greater expressive power in the language, Prolog began to include many of the capabilities of Planner that were left out of the original version of Prolog. References Bibliography Bruce Anderson. Documentation for LIB PICO-PLANNER School of Artificial Intelligence, Edinburgh University. 1972 Bruce Baumgart. Micro-Planner Alternate Reference Manual Stanford AI Lab Operating Note No. 67, April 1972. . . . . . Carl Hewitt. "The Challenge of Open Systems" Byte Magazine. April 1985 Carl Hewitt and Jeff Inman. "DAI Betwixt and Between: From ‘Intelligent Agents’ to Open Systems Science" IEEE Transactions on Systems, Man, and Cybernetics. Nov/Dec 1991. Carl Hewitt and Gul Agha. "Guarded Horn clause languages: are they deductive and Logical?" International Conference on Fifth Generation Computer Systems, Ohmsha 1988. Tokyo. Also