23 Some New Directions in Robot Problem Solving R. E. Fikes, P. E. Hart and N. J. Nilsson

An operator is characterized by a precondition statement describing the conditions under which it may be applied, and lists of statements describing its effects. Specifically, the effect of an operator is to remove from the model all statements matching forms on the operator's'delete list', and to add to the model all statements on the operator's'add list'.