How to use the pyaml.__init__.PrettyYAMLDumper.represent_undefined function in pyaml

'_-_'.join(map(op.attrgetter('value'), hint)) ) )
		else:
			self.anchors[node] = None
			if isinstance(node, yaml.nodes.SequenceNode):
				for item in node.value:
					self.anchor_node(item)
			elif isinstance(node, yaml.nodes.MappingNode):
				for key, value in node.value:
					self.anchor_node(key)
					self.anchor_node(value, hint=hint+[key])

PrettyYAMLDumper.add_representer(dict, PrettyYAMLDumper.represent_dict)
PrettyYAMLDumper.add_representer(defaultdict, PrettyYAMLDumper.represent_dict)
PrettyYAMLDumper.add_representer(OrderedDict, PrettyYAMLDumper.represent_odict)
PrettyYAMLDumper.add_representer(set, PrettyYAMLDumper.represent_list)
PrettyYAMLDumper.add_representer(None, PrettyYAMLDumper.represent_undefined)

if sys.version_info.major >= 3:
	try: import pathlib
	except ImportError: pass
	else:
		PrettyYAMLDumper.add_representer(
			type(pathlib.Path('')), lambda cls,o: cls.represent_data(str(o)) )


class UnsafePrettyYAMLDumper(PrettyYAMLDumper):

	def expect_block_sequence(self):
		self.increase_indent(flow=False, indentless=False)
		self.state = self.expect_first_block_sequence_item

	def expect_block_sequence_item(self, first=False):