Abstract. Formal specifications play an increasingly important role in system design-flows. Yet, they are not always easy to deal with. In this paper we present RATSY, a successor of the Requirements Analysis Tool RAT. RATSY extends RAT in several ways. First, it includes a new graphical user interface to specify system properties as simple Büchi word automata. Second, it can help debug incorrect specifications by means of a game-based approach. Third, it allows correct-by-construction synthesis of systems from their temporal properties. These new features and their seamless integration assist in property-based design processes.

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Deriving a formal specification from an informal design intent is an error-prone process. The resulting specification may be incomplete, unrealizable, or in conflict with the design intent. We propose a debugging method for incorrect specifications that does not need an implementation.We show that we can explain conflicts with the design intent by explaining unrealizability. Our approach for explaining unrealizability is based on counterstrategies. Since counterstrategies may be large, we propose several ways to simplify them. First, we simplify the specification itself by removing both requirements and variables that do not contribute to the problem. Second, we heuristically search for a countertrace, i.e., a single input trace that suffices to demonstrate unrealizability. Finally, we present the countertrace or the counterstrategy to the user in extensive form as a graph and implicitly as an interactive game. We present experimental results for specifications given as GR(1) formulas.

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