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XPath is a simple language for navigating an XML document and selecting a set of element nodes. XPath expressions are used to query XML data, describe key constraints, express transformations, and reference element s in remote documents. This paper studies the containment and equivalence problems for a fragment of the XPath query language, with applications in all these contexts. In particular, the authors study a class of XPath queries that contain branching, label wildcards and can express descend ant relationships between nodes. Prior work has shown that languages which combine any two of these three feat tires have efficient containment algorithms. However, the authors show that for the combination of features, containment is coNP - complete.
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