This is most often noticed in systems like widget hierarchies. By making both Widget and Window inherit from a class, then you can control which attributes get acquired. Widget's color is blue Note that a Widget is not a subclass of a Window, yet instances of it 'inherit' the color attribute. These are mutually exclusive mixin classes. Attribute setting propagates automatically at attribute setting time from containers to their parts, according to user-settable rules.
Instances can override the subscribed values.