Backtracking actions are interesting for the study of reasoning strategies. The following axioms allow us to take into account backtracking actions within our formalization. Axiom describes two simple heuristics for determining the appropriateness of backtracking actions, i.e., for detecting dead-ends during the searching process. Backtracking actions are taken when: (1) the length of the mental situation explored is equal to some threshold N ; or (2) its length is smaller than N but there are no appropriate mental actions that can be applied to it.
Figure: This figure illustrates the definition of Path(m) in axiom : , , , and .
It is possible to define more interesting heuristics for the appropriateness of backtracking actions, which detect early on unappropriate choices of the search strategy.
The path associated with a mental situation m describes the sequence of mental actions (including backtracking actions of the form Back) STRIPS needs to perform in order to find m (see figure for a graphical explanation).
Taking into account backtracking actions (i.e., including axioms , and ) requires modifying the definition of goal stack derivation (axiom ) as follows.