free of conditional logics previously needed for distinguishing between different kinds of abstractions and can thus more easily be incremen­ tally extended to support new kinds of abstractions. In other words, classes and inheritance are means to properly model variations of behavior related to the existence of different kinds of an abstract data type. The support for extensi­ bility and reuse with respect to such kind-specific behavior variations is among the main reasons for the increasing popularity of object-oriented programming in the last two decades. However, this popularity does not prevent us from questioning the real effec­ tiveness of current object-oriented techniques in supporting incremental vari­ ations. In fact, this popularity makes a critical investigation of the variations that can actually be performed incrementally even more important.