We describe CAVEAT, a technique and a tool (under development) for the stepwise design and verification of nearly finite-state concurrent systems (NFCS). A concurrent system is nearly finite-state when most of its variables have a finite range (Booleans, bounded integers). The heart of CAVEAT is a tool for verifying invariants, i.e., inductive safety properties. The underlying method is classical : formula I is an invariant for system ,S if and only if some formula ~r =def {I}S{I} is valid. If ,S is an NFCS, the formula ~5i contains only a small set of non-boolean variables. CAVEAT uses the connection method to extract from ~I a (small) set ~ of paths (some kind of assertions) about the non-boolean variables; ~5i is valid if and only if all paths contain connections, i.e., are inconsistent. For typical NFCS given with a correct invariant, the formula ~I is rather large (more than 100 lines) but k~ is quite small (a dozen one-line formulas). The second part of CAVEAT (not implemented yet) supports an incremental development method that is fairly systematic, but has proved to be flexible enough in practice.
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