Hierarchical task network (HTN) planning is a model-based approach to planning. The HTN domain model consists of tasks and methods to decompose them into subtasks until obtaining primitive tasks (actions). There are recent methods for verifying if a given action sequence is a valid HTN plan. However, if the plan is invalid, all existing verification methods only say so without explaining why the plan is invalid. In the paper, we propose a method that corrects a given action sequence to form a valid HTN plan by deleting the minimal number of actions. This plan correction explains what is wrong with a given action sequence concerning the HTN domain model.
In this paper, we consider the plan verification problem for totally ordered (TO) HTN planning. The problem is proved to be solvable in polynomial time by recognizing its connection to the membership decision problem for context-free grammars. Currently, most HTN plan verification approaches do not have special treatments for the TO configuration, and the only one features such an optimization still relies on an exhaustive search. Hence, we will develop a new TOHTN plan verification approach in this paper by extending the standard CYK parsing algorithm which acts as the best decision procedure in general.
Plan recognition is the problem of recognizing a goal task and an agent’s plan based on the observed actions. Plan recognition techniques can be employed in multiagent systems, behaviour recognition, computer security, and other fields related to artificial intelligence. Hierarchical task networks (HTN) describe the decomposition hierarchy of tasks in planning problems. In HTN plan recognition, a prefix of the plan (actions observed so far) is given as an input, and the aim is to find a task that decomposes into a sequence of actions with the given prefix. In this paper, we show how the performance of parsing-based HTN plan recognition can be improved by restricting possible suffixes of the given prefix based on generalized arc consistency of a corresponding context-free grammar.
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