Fig. 1. Left: Upward compatibility—dependencies on the base level need to be addressed at the meta level. Right: Downward compatibility—dependencies on the meta level need to be addressed at the base level.

Fig. 2. Property Composition and Property Application: two different stages in the process of reusing class properties.

Fig. 3. Metaclasses are composed from traits representing class properties. Traits supports upward and downward compatibility.

Fig. 4. Left. The trait TSingleton. Right. The class Behavior, the root of metaclasses in Smalltalk, is composed from the trait TCreator and as such provides the method basicNew.

Fig. 5. Left: Boolean hierarchy refactored with traits. Right: The complete picture for the Boolean hierarchy solution.

Fig. 6. A fine-grained architecture of class properties based on traits.

Fig. 7. Hierarchy of the new Smalltalk kernel supporting traits.

Fig. 8. Left: Smalltalk addresses compatibility issues by preventing reuse using implicit metaclasses and parallel hierarchies. Right: By default CLOS addresses compatibility issues by preventing subclasses to have different metaclasses than their superclasses.

Fig. 9. Left: SOM supports upward compatibility by automatically deriving new metaclasses and changing the class of the inheriting class B. Right: SOM downward compatibility failure example.

Fig. 10. Assigning the property m to class Meta B and property n to the class Meta A in NeoClasstalk. The light grey area denotes the metaclass area. The dark grey area is the realm of the class properties.

Fig. 11. Controlling class property propagation and trait composition.

Comparison of the models from Section 8 on how they handle the composition problems described in Section 2