def check_for_common_label(listInst):
	"""Return the boolean label shared by all instances in the given list of
	instances, or None if no such label exists

	>>> check_for_common_label([Instance([],True), Instance([],True)])
	True
	>>> check_for_common_label([Instance([],False), Instance([],False)])
	False
	>>> check_for_common_label([Instance([],True), Instance([],False)])"""

	instNum = len(listInst)
	posNum = len([inst for inst in listInst if inst.fLabel == True])
	if posNum == instNum:	return True
	elif posNum == 0:		return False
	return None