
This parses the argument, which is expected to be a well formed XML
fragment and returns a parse tree as a special object with underlying disk structure, named
"persistent XML" or "XPER"
While the result of xml_tree is a memory-resident array of vectors,
the XPER object consumes only a little amount of memory, and almost all data are disk-resident.
XPERs are better then "XML trees" for large documents and
for "write once -- read many" stores such as a table with
one XML document per row used as a "library" of documents.
To be saved in a LONG VARCHAR column, "XML tree" entity will be
converted back to plain text of XML syntax; but "XPER" entity
will be saved as a ready-to-use disk structure.
	