The formal description methods introduced in this paper were related to our simple language SPL for explanation purposes. Actually, the methods (i.e., abstract trees, predicates, state components, control trees, etc.) apply to all programming languages. However, some programming languages contain facilities that cannot be expressed using the methods described in this paper alone. For example, additional definitions are required to represent the multitasking facilities of PL/1. To solve this particular problem, an extension to the execution control of the abstract interpreter was used in the formal description of PL/1.
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