* {@snippet :
 * public class SafeBox<V> {
 *   private V value;
 *   private final Monitor monitor = new Monitor();
 *   private final Monitor.Guard valuePresent = monitor.newGuard(() -> value != null);
 *   private final Monitor.Guard valueAbsent = monitor.newGuard(() -> value == null);
 *
 *   public V get() throws InterruptedException {
 *     monitor.enterWhen(valuePresent);
 *     try {
 *       V result = value;