Automated planning is a central area of artificial intelligence, involving the design of languages and computational models for reasoning about actions, change and time. In domain-independent planning, a planning problem is specified by the description of an initial world state, a set of desired goals to achieve and a set of possible actions or action schemata (domain operators). A solution of a planning problem is a (partially) ordered set of actions forming a valid plan, whose execution in the initial state transforms it into a world state where the problem goals are satisfied.