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The design of an object-oriented database system is based on principles from semantic data models, logic databases, and expert systems. The basic concept is that of relationships under constraints - first, between objects and, secondly, between schemas, where they serve as loose links between them. Relationships are binary and are expressed by rules that contain conditions that ensure their validity at all times. Retrieval is also based on relationships and is accommodated by a logical query language based on Horn clauses. The existence of direct double links between object instances greatly simplifies the evaluation of recursive queries. Another important aspect is that of rules that, apart from relationships, can express other types of events that may take place only when certain conditions are met, such as aggregations. The proposed system is suitable for storage of data in such areas as computer-aided design and manufacturing, office information systems, and artificial intelligence.