Returns the previous NSXMLNode object in document order.
You use this method to “walk” backward through the tree structure representing an XML document or document section. (Use nextNode to traverse the tree in the opposite direction.) Document order is the natural order that XML constructs appear in markup text. If you send this message to the first node in the tree (that is, the root element), nil is returned. NSXMLNode bypasses namespace and attribute nodes when it traverses a tree in document order.