Proper Placement of Derived Classes in the Class Hierarchy

Users may derive new classes by defining views1 based on the current database contents. Some virtual classes are classified as brothers of existing classes, and others are either superclasses or subclasses of existing base and virtual classes. A base class is defined directly by the user using class definition constructs. A virtual class is classified as a brother of another class if it is derived from the latter class via a selection. To have a homogeneous system, virtual classes must be treated as first-class citizens in an object-oriented model.