VHPOP is a partial order planner loosely based on UCPOP. It draws from the experience gained in the mid 1990's on flaw selection strategies for partial order planning, and combines this with more recent developments in the field of domain independent planning such as distance based heuristics and reachability analysis.
The key strength of VHPOP is its versatility. It implements all the common flaw selection strategies, such as DUnf and DSep, LCFR, and ZLIFO. Many novel flaw selection strategies are implemented as well, e.g. ones selecting open conditions based on heuristic cost. Because no single flaw selection strategy seems to be the best for all problems, VHPOP allows several flaw selection strategies to be used simultaneously in a round-robin scheme. This makes it possible to combine the strengths of different strategies, allowing VHPOP to solve more problems.
VHPOP is fully compatible with level 1 of PDDL+. There is also support for planning with durative actions (level 3 of PDDL+), and this is accomplished by adding a simple temporal network (STN) to the regular plan representation of a partial order planner. The STN records temporal constraints between actions in a plan, and supersedes the simple ordering constraints usually recorded by partial order planners. The use of STNs permits actions with interval constraints on the duration.
VHPOP can work with either ground or lifted (partially instantiated) actions. For the planning competition we used only ground actions.