def _get_id(mf, url=None):
	"""
	get the uid of the mf object

	Args:
		mf: python dictionary of some microformats object
		url: optional URL to use in case no uid or url in mf

	Return: string containing the id or None
	"""

	props =  mf['properties']

	if 'uid' in props:
		return props['uid'][0]
	elif 'url' in props:
		return props['url'][0]
	else:
		return None