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Fluent Merging : A General Technique to Improve Reachability Heuristics and Factored Planning

Fluent merging is the process of combining two or more fluents (state variables) into a single “super” fluent. By compiling in some of the inter-fluent interactions, fluent merging can (1) help improve informedness of relaxed reachability heuristics, and (2) improve the efficiency of factored planning as it removes some of the intervariable dependencies. Although some special cases of fluent merging have been around (under other names), the technique in its full generality has not been exploited or analyzed. In this paper, we discuss the general motivations for and tradeoffs in fluent merging. We will argue that existing techniques are too conservative in identifying mergeable fluents. We will then provide some novel techniques based on causal graph analysis for identifying mergeable fluents.