Splicing Plans to Achieve Misordered Goals

Most paral lel planners are sensitive to the order in which goals and act iv i ty preconditions are specified. A "wrong" ordering can easily cause a solution to be missed. Permuting goals and preconditions on fa i lure in hopes of f inding a soluble order is in general computationally unacceptable. Plan spl icing is a solution to this problem. Splicing is a violent conf l ic t resolution procedure which involves the cutting of assertion dependencies, recursive demotion or excision of selected ac t iv i t ies around the cut, and reinsert ion of deachieved goals back into the middle of the planner's goal stack so that they can be replanned later to mend the plan around the splice. In a temporal planner, after an excision it is further necessary to relieve the "temporal stress" induced on surviving ac t i v i t ies by the ac t iv i t ies which were excised. This is an important capability for two reasons: f i r s t , because the order of achievement can of course not always be known in advance, and secondly because it is desirable to be able to present goals in pr io r i ty order.