The SIPE-2 planner developed at SRI [] includes a simple knowledge representation system called the Sort Hierarchy. The nodes in the SIPE-2 sort hierarchy are implemented as defstructs, but can be viewed as a simple type of frame. An implementation of the GFP exists for the Sort Hierarchy.
The behaviors supported by the Sort Hierarchy back end are as follows:
Use KB class sipe-kb to build SIPE-2 KBs.