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As an example of how a multigoal problem is stored,
consider the problem contained in Figure 13
where three packages, OB1, OB2
and OB3, are to be transported to
the same destination location, l_{d}. Initially the goal set
contains the goal of transporting
OB1 alone, represented as (ATOB OB1 l_{d}),
and the successful derivation is stored as Case A.
The second goal is then added to the set.
Since the problem just attempted achieves the first goal through a
decision sequence which has to be backtracked over
in order to solve the additional goal,
a second derivation, Case B, is stored.
This new derivation then solves the mutually interacting goals,
(ATOB OB1 l_{d}) and (ATOB OB2 l_{d}). Problemsolving then
continues with the addition of the third goal.
This goal is solved through simple extension of
the previous decision sequence. No case is stored which includes this goal.
This means that we have two cases stored in the library:
Case A corresponding to a singlegoal problem and Case B corresponding
to a multigoal problem containing two negatively interacting goals.
Multigoal problems are stored only when the problem goals
are mutually interacting, that is, only when their individual
derivations cannot be sequenced and extended to solve the full problem.
With DERSNLP+EBL's storage strategy, the size of the library is limited
by the amount of interaction in the domain.
For example, if there is no negative interaction, then only single
goal cases will be stored.
In the logistics transportation domain, there is
a potential for all problem goals to interact negatively.
However, since there are also a significant
percentage of noninteracting goals,
this strategy reduces the size of the library in comparison
to one in which all of the multigoal problems which
are successfully solved are stored.
This storage strategy also represents a tradeoff
since effort must be expended in merging the retrieved
cases into a full solution (See Section 3.3).
Figure 14:
A Library fragment indexing
stored cases which
solve a single input goal, (ATOB OB1 l_{d}).

Next: Indexing on the Basis
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Previous: Building the Case Library
11/5/1997