On Removing Left Recursion from Totally-ordered HTN Domain Models

Abstract

Hierarchical planning is a knowledge-based approach to automated planning, where high-level (compound) tasks are decomposed into simpler tasks until primitive tasks - actions - forming the plan are obtained. Hierarchical Task Networks (HTN) is a modeling formalism for hierarchical planning, where task decompositions are described using decomposition methods with specific constraints. These decomposition methods are very close to rewriting (production) rules of context-free grammars, specifically when the domain is totally ordered. Recursive rules, when a task decomposes to itself, are generally allowed. Generating plans from left to right brings advantage of the first actions in the plan to be ready for execution even if the plan is not yet fully generated. However, left recursion goes against this idea as it tends to generate the plan from right (end) till left (beginning). The paper shows how to remove left recursion from HTN decomposition methods using the ideas from the Greibach Normal Form.