win.add(i, new Message());
System.out.println("win = " + win);
assert win.size() == 5;
win.ack(1);
System.out.println("win = " + win);
assert win.size() == 4;
win.ack(4);
win.add(i, new Message());
System.out.println("win = " + win);
assert win.size() == 10;
win.ack(7);
assert win.size() == 3;
}