  The research in hierarchical planning has made considerable progress in the
last few years. Many recent systems do not rely on hand-tailored advice anymore
to find solutions, but are supposed to be domain-independent systems that come
with sophisticated solving techniques. In principle, this development would
make the comparison between systems easier (because the domains are not
tailored to a single system anymore) and -- much more important -- also the
integration into other systems, because the modeling process is less tedious
(due to the lack of advice) and there is no (or less) commitment to a certain
planning system the model is created for. However, these advantages are
destroyed by the lack of a common input language and feature set supported by
the different systems. In this paper, we propose an extension to PDDL, the
description language used in non-hierarchical planning, to the needs of
hierarchical planning systems. We restrict our language to a basic feature set
shared by many recent systems, give an extension of PDDL's EBNF syntax
definition, and discuss our extensions with respect to several planner-specific
input languages from related work.
