
This parses the argument, which is expected to be a well formed XML 
fragment and returns a parse tree as a special memory-resident object.
While xper_doc
creates some disk-resident data structure, xtree_doc() will work
faster but it may require more memory.
You may wish to use xtree_doc for small documents (e.g. less than
5 megabytes and xper_doc for larger documents.
	