public void putBeepers(int x, int y, int num) {
		Coordinate c = new Coordinate(x, y);
		if (num == Display.INFINITY) {
			synchronized (beepers) {
				beepers.put(c, new BeeperStack(x, y, num));
			}
			return;
		}
		synchronized (beepers) {
			int oldBeepers = 0;

			BeeperStack b;
			if ((b = beepers.get(c)) != null)
				oldBeepers = b.getBeepers();

			if (oldBeepers == Display.INFINITY)
				return;

			if (oldBeepers + num < 1)
				beepers.remove(c);
			else
				beepers.put(c, new BeeperStack(x, y, num + oldBeepers));
		}
	}