Simula 67 is a general purpose language developed at the Norwegian Computing Center. It evolved from an earlier simultation language called Simula 1 [1] Dahl and Nygaard however, realized that the problems that had to be solved for the implementation of a simulation language, namely the handling of complex data structures and of quasi-parallelism, were general problems. The language Simula 67 [2,3] was therefore defined as a general purpose language with a mechanism for extensions. The simulation capabilities are thus not defined in the base language Simula 67 but are rather defined by extension. This shows clearly how the language Simula 67, hereafter called Simula, is related to the idea of extensibility.