Prodigy planning algorithm

We present a formal description of the planning algorithm used in the Prodigy4.0 system. The algorithm is based on an interesting combination of backward-chaining planning and simulation of plan execution. The backward-chainer selects goal-relevant operators, and then Prodigy simulates their application to the current state of the world. The system can use different backward-chaining procedures, some of which are presented in the paper.