Most existing verification tools for process algebras allow the correctness of specifications to be checked in a fully automatic fashion. These systems have the obvious advantage of being easy to use, but unfortunately they also have some drawbacks. In particular, they do not always succeed in completing the verification analysis, due to the problem of state explosion, and they do not provide any insight into the meaning of the intended specifications. In this paper we consider an alternative

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... proach in which both interactive and automatic techniques are combined in the hope that the advantages of automation are retained, and that some of its disadvantages are overcome. To achieve our goal, we use the interactive theorem prover ItOL as a framework for supporting the theory of observational congruence of ccs, and provide a set of automatic proof tools, based on the algebraic axiomatization of the language, which can be used interactively. To illustrate how interaction and automation can be intermixed, we describe two verification strategies which exhibit different degrees of user interaction.