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Operator Tables

Aliases: Triangle Table

Keywords: preconditions

Categories: Planning

Author(s): Maarten van Someren

When Macro Operators are built from elementary Operators, the original operators and the macro operator are available, but not any sub-sequences of the macro operator. For example, if O(20) was built from O(1) through O(10), then it is not possible to use the sub-sequence O(8) through O(10). The representation in the figure accompanying this entry was devised to overcome this inefficiency. The preconditions of each operator are in the row preceding it, while the postconditions are placed in the columns below. The execution of subsequent operators may affect these postconditions, so they are updated in subsequent rows of the table by considering each operator's delete list.

Now, if we define the th kernel of the table as the intersection of all rows below and including the th and all columns to the left and including the th, we can easily verify whether the th action in an operator table is currently executable by checking whether all the predicates contained in the th kernel are true. Giving a planner a library of operator tables can reduce the search on new problems (see Library-based Planning). However, the entries in the tables should be generalised (with respect to variable bindings) if they are to be of any use.



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