We identify three programming language abstractions for the construction of reusable components: abstract type members, explicit selftypes, and symmetric mixin composition. Together, these abstractions enable us to transform an arbitrary assembly of static program parts with hard references between them into a system of reusable components. The transformation maintains the structure of the original system. We demonstrate this approach in two case studies, a subject/observer framework and a

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... ramework and a compiler front-end. All three abstractions have their theoretical foundation in the νObj calculus [OCRZ03]. They have been defined and