In this section we explain how XII utilizes LCW to satisfy universally quantified goals. Traditionally, planners that have dealt with goals of the form ``Forall of type t make true'' have done so by expanding the goal into a universally-ground, conjunctive goal called the universal base. The universal base of such a sentence equals the conjunction in which the s correspond to each possible interpretation of under the universe of discourse, , i.e. the possible objects of type t [][p. 10]genesereth-text. In each , all references to have been replaced with the constant . For example, suppose that pf denotes the type corresponding to the files in the directory /papers and that there are two such files: and . Then the universal base of ``Forall of type pf make printed() true'' is printed(a.dvi)printed(b.dvi).
A classical planner can satisfy goals by subgoaling to achieve the universal base, but this strategy relies on the closed world assumption. Only by assuming that all members of the universe of discourse are known (i.e., represented in the model) can one be confident that the universal base is equivalent to the goal. Since the presence of incomplete information invalidates the closed world assumption, the XII planner uses two new mechanisms for satisfying goals:
For completeness, XII also considers combinations of these mechanisms to solve a single goal; see [8] for details. In the remainder of this section we explain these two techniques in more detail.