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<p>The most important thing to keep in mind when using XQuery Update is the Pending Update List (PUL). In contrast to other programming languages, XQUF statements are not executed immediately, but are first collected in within a set-like structure, the PUL. At the end of a query, all update primitives on this list are applied after being checked for compatibility. If a conflict exists, an error message is returned and all accessed databases remain untouched (atomicity).