* collection that is currently being modified by another thread.
   *
   * @throws ClassCastException if the elements are not mutually comparable
   * @throws NullPointerException if any of {@code elements} is null
   * @since 7.0 (source-compatible since 2.0)
   */
  public static <E> ImmutableSortedSet<E> copyOf(Collection<? extends E> elements) {
    // Hack around E not being a subtype of Comparable.

  private static final LazyLogger log = new LazyLogger(SequentialExecutor.class);

  enum WorkerRunningState {
    /** Runnable is not running and not queued for execution */
    IDLE,
    /** Runnable is not running, but is being queued for execution */
    QUEUING,
    /** runnable has been submitted but has not yet begun execution */
    QUEUED,
    RUNNING,
  }

  /** Underlying executor that all submitted Runnable objects are run on. */

   * Returns a predicate that evaluates to {@code true} if the object reference being tested is
   * null.
   */
  @GwtCompatible(serializable = true)
  public static <T extends @Nullable Object> Predicate<T> isNull() {
    return ObjectPredicate.IS_NULL.withNarrowedType();
  }

  /**
   * Returns a predicate that evaluates to {@code true} if the object reference being tested is not
   * null.
   */
  @GwtCompatible(serializable = true)

   * delegating calls until it returns valid data.
   *
   * @param duration the length of time after a value is created that it should stop being returned
   *     by subsequent {@code get()} calls
   * @param unit the unit that {@code duration} is expressed in
   * @throws IllegalArgumentException if {@code duration} is not positive
   * @since 2.0
   */

   * finally block.
   *
   * Further optimizations of this algorithm become increasingly subtle. A wait that terminates
   * without the guard being satisfied (due to timeout, but not interrupt) can then immediately exit
   * the monitor without signalling. If it timed out without being signalled, it does not need to
   * "pass on" the signal to another thread. If it *was* signalled, then its guard must have been

  }

  /**
   * Wrapper for {@link Map#get(Object)} that forces the caller to pass in a key of the same type as
   * the map. Besides being slightly shorter than code that uses {@link #getMap()}, it also ensures
   * that callers don't pass an {@link Entry} by mistake.
   */
  protected V get(K key) {
    return getMap().get(key);
  }

    }
  }

  /**
   * Starts a new task on the given executor that switches (deletes and replaces) a file between
   * being a directory and being a symlink. The given {@code file} is the file that should switch
   * between being a directory and being a symlink, while the given {@code target} is the target the
   * symlink should have.
   */
  private static void startDirectorySymlinkSwitching(

   */
  // Note: 90% of the time we don't add type parameters or wildcards that serve only to "tweak" the
  // desired return type. However, *nested* generics introduce a special class of problems that we
  // think tip it over into being worthwhile.
  public static <T extends @Nullable Object, S extends T> Comparator<Iterable<S>> lexicographical(
      Comparator<T> comparator) {
    return new LexicographicalOrdering<S>(checkNotNull(comparator));
  }

    /** Lock used whenever accessing the state variables (runningTasks, shutdown) of the executor */
    private final Object lock = new Object();

    /*
     * Conceptually, these two variables describe the executor being in
     * one of three states:
     *   - Active: shutdown == false
     *   - Shutdown: runningTasks > 0 and shutdown == true
     *   - Terminated: runningTasks == 0 and shutdown == true
     */

   * collection that is currently being modified by another thread.
   *
   * @throws ClassCastException if the elements are not mutually comparable
   * @throws NullPointerException if any of {@code elements} is null
   * @since 7.0 (source-compatible since 2.0)
   */
  public static <E> ImmutableSortedSet<E> copyOf(Collection<? extends E> elements) {
    // Hack around E not being a subtype of Comparable.