<?xml version="1.0" standalone="yes"?>
<Paper uid="E89-1005">
  <Title>A METAPLAN MODEL FOR PROBLEM-SOLVING DISCOURSE*</Title>
  <Section position="3" start_page="0" end_page="0" type="intro">
    <SectionTitle>
2. PLAN BUILDING METAPLANS
</SectionTitle>
    <Paragraph position="0"> In this approach, the plan-building metaplans discussed in this section model those portions of problem-solving behavior that explore the different possible refinements of the plan being considered and the different possible variable instantiations for it. The domain for all the examples in this paper is naval operations, where the agent is assumed to be a naval officer and the expert a cooperative interface to a fleet information system. The examples assume a scenario in which a particular vessel, the Knox, has been damaged in an accident, thereby lowering its readiness and that of its group. The top-level goal is thus assumed to be restoring the readiness of that group from its current poor rating to good, expressed as (IncreaseGroup-Readiness Knox-group poor good).</Paragraph>
    <Paragraph position="1"> The domain plans in Pragma are organized in a classification hierarchy based on their effects and preconditions, so that a node in that hierarchy like the top-level instance of IncreaseGroupReadiness in the examples actually stands for the class of plans that would achieve that result in a certain class of situations. The plan class nodes in this hierarchy can thus be used to represent partially specified plans, the set of plans that an agent might be considering that achieves a particular goal using a particular strategy. The subplans (really plan subclasses) of IncreaseGroupReadiness shown in Figure 1 give an idea of the different strategies that the agent may consider for achieving this goal.</Paragraph>
    <Paragraph position="2"> (Variables are shown with a prefixed question</Paragraph>
    <Paragraph position="4"> The plan classification depends on the circumstances, so that RepairShip only functions as a subplan of IncreaseGroupReadiness when its object ship is specified as the Knox, the damaged one, but some of the plans also introduce new variables like ?new-ship, introduced by the ReplaceShip plan, that can take on any value permitted by the plan's preconditions.</Paragraph>
    <Paragraph position="5"> Each of these plans also has its own subactions describing how it can be achieved, so that ReplaceShip, for example, involves sailing the ?new-ship to the location of the damaged ship, having it take over the duties of the damaged ship, and then sailing or towing the damaged one to a repair facility. Those subactions, in turn, specify goals for which there can be multiple subplans. The metaplan structures modeling the problem-solving discourse are built on top of this tree of domain plans and actions.</Paragraph>
    <Paragraph position="6">  The build-plan metaplan is used to capture the agent's goal of constructing a plan to achieve a particular goal, with the build-subplan and build-subaction metaplans modeling the problem-solving steps that the agent uses to explore and refine the class of domain plans for that goal. An instance of build-subplan, say, reflects the agent's choice of one of the possible subplan refinements of the current domain plan as the candidate plan to be further explored. For example, the initial context assuming an lncreaseGroupReadiness plan due to damage to the Knox would be represented in our model by the build-plan node on line (1) of Figure 2.</Paragraph>
    <Paragraph position="8"/>
    <Section position="1" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
and Build-Subaction
</SectionTitle>
      <Paragraph position="0"> If we suppose that the agent first considers replacing Knox with some other frigate, that would be modeled as a build.subplan child (2) of the build.plan for the IncreaseGroupReadiness plan (1), that would in turn generate a new build-plan for ReplaceShip (3). If the agent continues by considering how to get the new ship to that location, that would be represented as a build-subaction child (4) of the buiM-plan for ReplaceShip that expands the Sail action.</Paragraph>
      <Paragraph position="1"> - 36 -</Paragraph>
    </Section>
    <Section position="2" start_page="0" end_page="0" type="sub_section">
      <SectionTitle>
Variable Constraining Metaplans
</SectionTitle>
      <Paragraph position="0"> In addition to the plan-refining choice of subplans and exploration of subactions, the other plan-building task is the instantiation of the free variables found in the plans. Such variables may either be directly instantiated to a specified value, as modeled by the instantiate-var metaplan, or more gradually constrained to sub-sets of the possible values, as modeled by add-constraint.</Paragraph>
      <Paragraph position="1"> The instantiate-var metaplan reflects the agent's choice of a particular entity to instantiate an open variable in the current plan. For example, the ReplaceShip plan in Figure 2 (3) introduces a free variable for the ?new-ship. If the agent were to choose the Roark as a replacement vessel, that would be modeled by an instantiate-var metaplan attached to the buiM-plan node that first introduced the variable, as shown in Figure 3.</Paragraph>
      <Paragraph position="3"> The agent may also constrain the possible values for a free variable without instantiating it by using a predicate to filter the set of possible fillers. For example, the agent might decide to consider as replacement vessels only those that are within 500 miles of the damaged one. The predicate from the add-constraint node in line (2) of Figure4 is inherited by the lower buiM-plan node (3), which thus represents the agent's consideration of the smaller class of plans where the value of ?new-ship satisfies the added constraint.</Paragraph>
      <Paragraph position="4">  The metaplan context tree thus inherits its basic structure from the domain plans as reflected in the build-plan, build-subplan, and build-subaction nodes, and as further specified by the instantiation of domain plan variables recorded in instantiate-var and add-constraint nodes. Because the domain plans occur as arguments to the plan-building metaplans, the metaplan tree turns out to include all the information that would be available from a normal domain plan context tree, so that no separate domain tree structure is needed.</Paragraph>
    </Section>
  </Section>
class="xml-element"></Paper>