def flatten(combo):
	"""
	Given a combo nested `tuple`, e.g., 
	((('LY', 'XR'), ('LWL', 'RUR'))
	return a list of labels contained in 
	the combo ['LY', 'XR', 'LWL', 'RUR']
	"""
	
	return_list = []
	for i in combo:
		if isinstance(i, tuple):
			return_list.extend(flatten(i))
		else:
			return_list.append(i)
	return return_list