'Update the XML element tree based on YAML data.  Override
this method to add elements to the XML output.  Create new
Element objects and add them to the xml_parent.  The YAML data
structure must not be modified.
:arg YAMLParser parser: the global YAML Parser
:arg Element xml_parent: the parent XML element
:arg dict data: the YAML data structure'

def gen_xml(self, xml_parent, data):

    pass
