Ordered Landmarks in Planning
  Many known planning tasks have inherent constraints concerning the best order
in which to achieve the goals. A number of research efforts have been made to
detect such constraints and to use them for guiding search, in the hope of
speeding up the planning process. We go beyond the previous approaches by
considering ordering constraints not only over the (top-level) goals, but also
over the sub-goals that will necessarily arise during planning. Landmarks are
facts that must be true at some point in every valid solution plan. We extend
Koehler and Hoffmann's definition of reasonable orders between top level goals
to the more general case of landmarks. We show how landmarks can be found, how
their reasonable orders can be approximated, and how this information can be
used to decompose a given planning task into several smaller sub-tasks. Our
methodology is completely domain- and planner-independent. The implementation
demonstrates that the approach can yield significant runtime performance
improvements when used as a control loop around state-of-the-art sub-optimal
planning systems, as exemplified by FF and LPG.
