
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 is disk-resident.
    
This function is equivalent to xper_doc, and the only
difference is in the order of arguments; xper_doc() has the same order of arguments as
xml_tree.
	