Virtual copies: at the boundary between classes and instances

Knowledge bases built in object-oriented systems use networks of interconnected objects in their representations. The mechanism described here provides a way to use such a network as a prototype by making virtual copies of it. The virtual copy is created incrementally. Values of instance variables in the virtual copy are inherited from the prototype until locally overridden in the copy, similar to inheritance of defaults between instances and classes in Loops. A virtual copy preserves the topology of the original network. Virtual copies can be made from virtual copies. Alternative implementations of virtual copies allow different tradeoffs in space and lookup time. Virtual copies can be used for building knowledge bases for design, for representing contexts in a problem solving system, and have other uses in ordinary programming.