+ (id)defaultCenter
{
	static MAKVONotificationCenter *center = nil;
	if(!center)
	{
		// do a bit of clever atomic setting to make this thread safe
		// if two threads try to set simultaneously, one will fail
		// and the other will set things up so that the failing thread
		// gets the shared center
		MAKVONotificationCenter *newCenter = [[self alloc] init];
		if(!OSAtomicCompareAndSwapPtrBarrier(nil, newCenter, (void *)&center))
			[newCenter release];
	}
	return center;
}