A knowledge base is consistent if all its data, information, or knowledge (regardless they are stored in database or implied by inference engine) satisfy the predefined constraints which represent a real world rules or restrictions. Maintaining knowledge consistency in an object-oriented knowledge base is difficult since (i) two data items in the knowledge base may have the relationship that is not explicitly represented by the inheritance; and (ii) logic programming trends to manipulate "flat" data. In contrast, it is easier to develp a mechanism for maintaining the consistency in a value-oriented system (e.g., relational database or logic programming) [Ullm87].