Constraints. Constraints represented as formulas can be used to provide implicit action preconditions (e.g., (GKL97)). Namely, the set A( s) of actions applicable in s will exclude all actions a that lead to states Sa = next( a, s) that violate a constraint. Such constraints can be used to express capacity constraints (e.g., that the number of balls being held cannot exceed the number of grippers) or control knowledge (BK98). With constraints, any Constraint Satisfaction Problem (CSP) can be expressed as a Planning problem. The question is how to make those constraints play an active role in the search. So far, only approaches based exclusively on constraint techniques are able to do that (e.g., (KS99; BC99)). A combination of constraint-directed and heuristic search techniques may also prove useful.