Gets the modified time of the file or its accompanying XML file,
        whichever is greater.

def _get_mod_mtime(self, filepath):
        """Gets the modified time of the file or its accompanying XML file,
        whichever is greater.
        """
        file_mtime = self.tramp.getmtime(filepath)        
        xmlpath = self.get_xmldoc_path(filepath)
        if self.tramp.exists(xmlpath):
            xml_mtime = self.tramp.getmtime(xmlpath)
            if xml_mtime > file_mtime:
                file_mtime = xml_mtime

        return file_mtime