The hierarchical and semistructured nature of XML data may cause complicated update-behavior. Updates should not be limited to entire document trees, but should ideally involve subtrees and even individual elements. Providing a suitable scheduling algorithm for semistructured data can significantly improve collaboration systems that store their data -- e.g. word processing documents or vector graphics-- as XML documents. In this paper we improve upon earlier work (see [5]) which presented two equivalent concurrency control mechanisms based on Path Locks. In contrast to the earlier work, we now provide details regarding the workings of a commit scheduler for XML databases which uses the path lock conflict rules. We also give a comprehensive proof of serializability which enhances and clarifies the ideas in our previous work.