In existing simulation proof techniques, a single step in a lower-level specification may be simulated by an extended execution fragment in a higher-level one. As a result, it is cumbersome to mechanize these techniques using general-purpose theorem provers. Moreover, it is undecidable whether a given relation is a simulation, even if tautology checking is decidable for the underlying specification logic. This article studies various types of normed simulations. In a normed simulation, each step in a lower-level specification can be simulated by at most one step in the higher-level one, for any related pair of states. In earlier work we demonstrated that normed simulations are quite useful as a vehicle for the formalization of refinement proofs via theorem provers. Here we show that normed simulations also have pleasant theoretical properties: (1) under some reasonable assumptions, it is decidable whether a given relation is a normed forward simulation, provided tautology checking is decidable for the underlying logic; (2) at the semantic level, normed forward and backward simulations together form a complete proof method for establishing behavior inclusion, provided that the higher-level specification has finite invisible nondeterminism.
A b s tr a c t. There is an overwhelming number of different proof tools available and it is hard to find the right one for a particular application. Manuals usually concentrate on the strong points of a proof tool, but to make a good choice, one should also know (1) which are the weak points and (2) whether the proof tool is suited for the application in hand. This paper gives an initial impetus to a consumers' report on proof tools. The powerful higher-order logic proof tools PVS and Isabelle are com pared with respect to several aspects: logic, specification language, prover, soundness, proof manager, user interface (and more). The paper con cludes with a list of criteria for judging proof tools, it is applied to both PVS and Isabelle.1994 Mathematics Subject Classification: 03B35 Mechanisation of proof and logical operations; 03B15 Higher-order logic and type-theory.
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