When debugging a distributed system, it is sometimes necessary to explain the absence of an event - for instance, why a certain route is not available, or why a certain packet did not arrive. Existing debuggers offer some support for explaining the presence of events, usually by providing the equivalent of a backtrace in conventional debuggers, but they are not very good at answering 'Why not?' questions: there is simply no starting point for a possible backtrace.
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Wu et al. (2014) studied this question.
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