Modify the current document or the document specified by the second document argument according to XUpdate commands of the first document argument. XUpdate, or more specifically the XML Update Language, aims to be a language for updating XML documents.
XUpdate language is described in XUpdate Working Draft at http://xmldb-org.sourceforge.net/xupdate/xupdate-wd.html.
XUpdate output can be generated for example by Python xmldiff utility from http://www.logilab.org/projects/xmldiff/.