This paper describes techniques used in a large and complex knowledge-based simulation with thousands of qualitative rules organized hierarchically, which requires recursive combining rules. By formalizing the concepts otherwise implicit in combining rules, the authors have been able to speed model development, communicate results, improve the quality of the rules, and make it easier to recognize when different types of combination rules are needed. The paper's discussion is merely illustrative, but, in the long run, there should be implications for rule algebras in formal modeling and new syntaxes in programming languages. The objective should be to state combining rules at a high level of abstraction.
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