25. The computer-readable storage medium of claim 23 , additionally comprising instructions which, when executed by the one or more processors, causes the one or more processors to perform: receiving a DTD or XML schema associated with the registered XPaths; wherein the tree that represents a structural hierarchy is a metadata tree representing the received DTD or schema, wherein nodes of said metadata tree matching registered XPaths are annotated with information about the associated registered XPath.
23. The computer-readable storage medium of claim 14 , additionally comprising instructions which, when executed by the one or more processors, causes the one or more processors to perform: compiling registered XPaths into a tree that represents a structural hierarchy for the registered XPaths.