code: def _add_meta_value_to_xml_doc(self, doc, parent, obj):
          
          return self._add_subtree_to_xml_doc(doc,
                                              parent,
                                              subtree=obj,
                                              key='meta',
                                              key_order=None)
docs: Values in the meta element dict are converted to a BadgerFish-style
              encoding (see _convert_hbf_meta_val_for_xml), so regardless of input_format,
              we treat them as if they were BadgerFish.