}
      }

      @Override
      public Entry<E> next() {
        if (!hasNext()) {
          throw new NoSuchElementException();
        }
        // requireNonNull is safe because current is only nulled out after iteration is complete.
        Entry<E> result = wrapEntry(requireNonNull(current));
        prevEntry = result;
        if (current.succ() == header) {
          current = null;
        } else {

 *
 * <p>If the precondition is not met, the {@code Preconditions} method throws an unchecked exception
 * of a specified type, which helps the method in which the exception was thrown communicate that
 * its caller has made a mistake. This allows constructs such as
 *
 * <pre>{@code
 * public static double sqrt(double value) {
 *   if (value < 0) {
 *     throw new IllegalArgumentException("input is negative: " + value);
 *   }

   * The {@link Iterators} analog to this method is {@link Iterators#getNext}.
   *
   * <p>If no default value is desired (and the caller instead wants a {@link
   * NoSuchElementException} to be thrown), it is recommended that {@code
   * iterable.iterator().next()} is used instead.
   *

   * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this
   * method throws an exception, whereas the Java 8+ method returns the {@code null} to the caller.
   *
   * @throws NullPointerException if this optional's value is absent and the supplier returns {@code
   *     null}
   */
  public abstract T or(Supplier<? extends T> supplier);

  /**

      thrownUnsafeFailure = unsafeFailure;
      // catch absolutely everything and fall through to our 'SafeAtomicHelper'
      // The access control checks that ARFU does means the caller class has to be AbstractFuture
      // instead of SafeAtomicHelper, so we annoyingly define these here
      try {
        helper =
            new SafeAtomicHelper(

   * {@code invokeAny}. In the latter case, tasks will run serially on the calling thread. Tasks are
   * run to completion before a {@code Future} is returned to the caller (unless the executor has
   * been shutdown).
   *
   * <p>Although all tasks are immediately executed in the thread that submitted the task, this

  //    visible to done(). This is because there is no happens before edge between the write and a
  //    (hypothetical) unsafe read by our caller. Note: adding 'volatile' does not fix this issue,
  //    it would just add an edge such that if done() observed non-null, then it would also

    return new MutableClassToInstanceMap<>(new HashMap<Class<? extends @NonNull B>, B>());
  }

  /**
   * Returns a new {@code MutableClassToInstanceMap} instance backed by a given empty {@code
   * backingMap}. The caller surrenders control of the backing map, and thus should not allow any
   * direct references to it to remain accessible.
   */
  public static <B extends @Nullable Object> MutableClassToInstanceMap<B> create(

   * Optional.orElseGet}, except when {@code supplier} returns {@code null}. In this case this
   * method throws an exception, whereas the Java 8+ method returns the {@code null} to the caller.
   *
   * @throws NullPointerException if this optional's value is absent and the supplier returns {@code
   *     null}
   */
  public abstract T or(Supplier<? extends T> supplier);

  /**

   *
   * @since 32.0.0
   */
  public static final String OBSERVE_BROWSING_TOPICS = "Observe-Browsing-Topics";

  /**
   * The HTTP <a
   * href="https://wicg.github.io/turtledove/#handling-direct-from-seller-signals">{@code
   * Sec-Ad-Auction-Fetch}</a> header field name.
   *
   * @since 33.0.0
   */
  public static final String SEC_AD_AUCTION_FETCH = "Sec-Ad-Auction-Fetch";

  /**
   * The HTTP <a