The idea of dependency directed backtracking proposed by Stallman and Sussman (1977)
offers significant advantages over heuristic starch schemes with chronological
backtracking which waste much effort by discarding many "good" choices when
backtracking situations arise. However, we have found that existing non-chronological
backtracking machinery is not suitable for certain types of problems, namely, those
where choices do not follow logically from previous choices, but are based on a heuristic
evaluation of a constrained set of alternatives. This is because a choice is not justified by
a âset of supportâ (of previous choices), but because its advantages outweigh its
drawbacks in comparison to its competitors. What is needed for these types of problems
is a scheme where the advantages and disadvantages of choices are explicitly recorded
during problem solving. Then, if an unacceptable situation arises, information about the
nature of the unacceptability and the tradeoffs can be used to determine the most
appropriate backtracking point. Further, this requires the problem solver to use its
hindsight to preserve those "good" intervening choices that were made chronologically
after the "bad" choice, and to resume its subsequent reasoning in fight of the modified
set of constraints. In this paper, we describe a problem solver for non-chronological
backtracking in situations involving tradeoffs. By endowing the backtracker with access
to domain-specific knowledge, a highly contextual approach to reasoning in dependency
directed backtracking situations can be achieved.Information Systems Working Papers Serie
