If the planner can post constraints on its planning variables, then the problem of unifying two variables can be combinatorial if the constraints are sufficiently expressive. A sufficiently expressive planning formalism requires deduction of context-dependent effects. Modifying a plan, while retaining as much as possible of the original plan, is an intractable problem. The search space of operator applications is combinatorial. Given this complexity, planners strive to balance epistemological and heuristic adequacy, i.e., retain as much expressive power as is practical yet make enough restricting assumptions so that an efficient implementation may still be realized.
Planning is viewed as a search through the space of operator applications and plan orderings.
No predicate changes its truth value when an event takes place unless the operator representing the event explicitly states that the truth value changes. These predicates are added to or deleted from the world description. This assumption is at the heart of the efficiency achieved by classical planners and of their limitations. It kept track of abstraction level by assigning each predicate a level number. It formed a complete plan, based on all the predicates with less than a given level number, before addressing predicates with higher level numbers.