Modify the current document or the document specified by the second id argument according to XUpdate commands of the first id document. XUpdate is a XML Update Language which aims to be a language for updating XML documents.
XUpdate langauge is described in XUpdate Working Draft at http://www.xmldb.org/xupdate/xupdate-wd.html.
XUpdate output can be generated for example by Python xmldiff utility from http://www.logilab.org/xmldiff/. Unfortunatelly, there are few bugs (or, as I tend to say In case of Python, white-space problems) in their code, so its XUpdate output is not always correct.