Non-deterministic formalisms are used to model systems whose runtime behaviour is inherently non-deterministic (its runtime execution might be different in consecutive runs, even for the same inputs). To analyse these systems, the full state space is explored to check whether an unwanted state (for example: deadlock, unsafe) can be reached. Debugging support for such formalisms is currently limited. This paper presents a prototype tool which allows to interactively construct the reachability

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... ph (by manually stepping). The construction can be automatically paused at the moment a state of interest is reached (breakpointing). This should lead to earlier detection of errors and easier resolution, since the user can observe and control the reachability analysis.