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XML data management using relational database systems has been intensively studied in the last few years. However, in order for such systems to be viable, they must support not only queries, but also updates over virtual XML views that wrap the relational base. While view updating is a long-standing difficult issue in the relational context, the recent invention of the flexible XML data model and nested XML query language poses new challenges for view updating. In this paper, we first formally model the XQuery View Update Problem. Then the clean extended-source theory for determining the existence of a relational update translation for a given XQuery view update is proposed. We identify new update translation issues, in particular view construction consistency, update granularity and view duplication, which are caused by the distinct features of XML, most notably, the XML hierarchical data structure and nested XQuery expressions. We propose a graph-based algorithm for detecting the occurence of these issues by view definition analysis. The connection between the algorithm with the clean extended-source theory is also established. Finally, the XML view updatability for various update granularity is presented according to different update types.