An example of hierarchical planning

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An example of hierarchical planning(1)
planning a sequence of actions- N.B. not yet
linguistic!
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A goal to be achieved by planning

• Task
• Ed wants to be in New York
• Formalization as a plan goal
• be-at (Ed, New York)
• The algorithm Top-down hierarchical planning
• Search our library of plan operators for ways of
  achieving the goal
• For an operator to be usable, the preconditions
  much match the state of the world
• For an operator to be useful, the effect must
  leave us nearer to achieving our goal than we
  were before!

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Do we have a plan operator with a useful effect?
be-at (Ed, New York)
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Our PLAN OPERATORS
?
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be-at (Ed, New York)
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be-at (Ed, New York)
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be-at (Ed, New York)
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Do we have a plan operator with a useful effect?
be-at (Ed, New York)
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Our PLAN OPERATORS
cost?time?
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This one looks promising, the planning process
decides to take a closer look
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We have a useful plan operator so we
instantiate the operator
Task goal be-at (Ed, NY)
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ACTOR Ed TRAIN?train DESTINATION NY
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Testing phase

• Do the preconditions hold?
• If not, we have to make them hold by planning
  further before we can carry out the action and
  get the desired effect
• When the preconditions hold, we can achieve the
  effect by adding the actions in the body into our
  plan. This is the hierarchical expansion of the
  plan tree.

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Do the preconditions hold?
ACTOR Ed TRAIN?train DESTINATION NY
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The preconditions do not yet match the world
unfortunately
PRECONDITIONS
WORLD
Destination (?train, NY) On-board (Ed, ?train)
Be-at (Ed, Washington)
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We set up some new goals (called posting) and
see if we can plan for them to be achieved
PRECONDITIONS
GOALS

Destination (?train, NY) On-board (Ed, ?train)

• Lets us assume that there are lots of trains to
  NY from Washington, so the first goal can be
  filled in directly (be satisfied) by looking in
  a database, e.g.
• destination (9am-train, NY)
• destination (10am-train ,NY)
• destination (11am-train, NY).

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We set up some new goals (called posting) and
see if we can plan for them to be achieved
PRECONDITIONS
GOALS
Destination (?train, NY) On-board (Ed, ?train)

• The second goal takes more work. We have to look
  in our plan library for operators with an effect
  that is relevant for
• On-board (Ed, 10am-train)

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Do we have a plan operator with a useful effect?
On-board (Ed, 10am-train)
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On-board (Ed, 10am-train)
?
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On-board (Ed, 10am-train)
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On-board (Ed, 10am-train)
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On-board (Ed, 10am-train)
This one looks promising, the planning process
decides to take a closer look
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ACTOR Ed TRAIN 10am-train
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Do the preconditions hold?
ACTOR Ed TRAIN10-am-train
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The preconditions do not yet match the world
unfortunately
PRECONDITIONS
WORLD
Be-at (Ed, 10am-train) Have (Ed, ticket
(10am-train))
Be-at (Ed, Washington)
Further planning as above
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Resulting in a final complete hierarchical plan
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A complete plan

• is a hierarchically organized sequence of actions
• carrying out the actions in sequence will
  guarantee that the preconditions for achieving
  and subsequent effect are met in time
• when all the sub-actions of a more abstract
  action have been performed, the abstract action
  has also been achieved and the corresponding
  effect also

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COMPLETE SUMMARY OF THE PLAN and PLAN LIBRARY
(Bateman Zock, 2003)