Operator Preconditions
Precondition: A necessary property of a state in which a particular operator can be applied.
Example: In checkers, a piece may only move into a square that is vacant.
Vacant(place) is a precondition on moving a piece into place.
Example: In Chess, a precondition for moving a rook from square A to square B is that all squares between A and B be vacant. (A an B must also be either in the same row or the same column.)