*
   * @author Luke Sandberg
   * @since 11.0
   */
  public abstract static class CustomScheduler extends Scheduler {
    /** Constructor for use by subclasses. */
    public CustomScheduler() {}

    /** A callable class that can reschedule itself using a {@link CustomScheduler}. */
    private final class ReschedulableCallable implements Callable<@Nullable Void> {

      /** The underlying task. */

   *
   * @author Luke Sandberg
   * @since 11.0
   */
  public abstract static class CustomScheduler extends Scheduler {
    /** Constructor for use by subclasses. */
    public CustomScheduler() {}

    /** A callable class that can reschedule itself using a {@link CustomScheduler}. */
    private final class ReschedulableCallable implements Callable<@Nullable Void> {

      /** The underlying task. */