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Wed 20 Jan 2016 15:35 - 16:00 at Grand Bay North - Track 1: Decision Procedures Chair(s): Loris D'Antoni

We show the diagonal problem for higher-order pushdown automata (HOPDA), and hence the simultaneous unboundedness problem, is decidable. From recent work by Zetsche this means that we can construct the downwards closure of the set of words accepted by a given HOPDA. This also means we can construct the downwards closure of the Parikh image of a HOPDA. Both of these consequences can play an important role in verifying concurrent higher-order programs expressed as HOPDA or safe higher-order recursion schemes.