The reuse of libraries of classes by client applications is an interesting issue quite difficult to achieve, especially when modification of the class tree is needed but not possible because of the context. We propose a solution which is based on the presence of both specialisation and generalisation relationships in an object-oriented programming language. The specification of both relationships is based on a meta-model called OFL which provides a support for describing the operational semantics of a language through the definition of parameters and semantical actions. We propose an overview of the expressiveness of OFL and of its implementation and we give also some other interesting applications.