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By Ageev A.A.

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And = 2 r K<0-*2(J)•2 »•He)s2(e)-T,{X\ ). d<=D e&D Applying RSP again yields rj(Xf\) = TJ(X\ ) and therefore This finishes the proof of the theorem. • REMARK. Channels K and L can contain only one datum at a time. Now one can say that this is no problem because S and R will never send a message into a channel when the previous one is still there. If S and R would do this then our process algebra modelling would be incorrect. Because they don't, there is no problem. This argument is correct for the ABP, but one should be careful in more complex situations: if one implicitly uses assumptions about the behaviour of a system in the specification of that system, then there is a danger that a verification shows that the system has a lot of 'wonderful' properties which in reality it has not.

Definition for channels K and L The reason why we use actions / and y, instead of the r as is done in the specification of the ABP, will become clear further on. 3. The sender. In the specification of the sender process S (Table 7) we use formal variables RH\ SF*1, STd\ WSdn (dzD, nsB): RH— Read a message from the Host at port 1. The host process, which is not specified here, furnishes the sender with data. SF — Send a Frame in channel K at port 3. ST= Start the Timer. WS = Wait for Something to happen.

We are interested in a technique which makes it possible to abstract from the internal structure of a system, so that we can derive statements about the external behaviour. Abstraction is an indispensable tool for managing the complexity of process verifications. This is because abstraction allows for a reduction of the complexity (the number of states) of subprocesses. This makes it possible to verify large processes in a hierarchical way. A typical verification consists of a proof that, after abstraction, an implementation IMP behaves like the much simpler process SPEC which serves as system specification: ABS(IMP) = SPEC In process algebra we model abstraction by making the distinction between two types of actions, namely external or observable actions and the internal or hidden action T, and by introducing explicit abstraction operators 17 which transform observable actions into the hidden action (see Figure 3).