private static final @Nullable String NE = null;

  private static <E extends @Nullable Object> Entry<E> entry(E element, int count) {
    return Multisets.immutableEntry(element, count);
  }

  private static <E extends @Nullable Object> Entry<E> control(E element, int count) {
    return HashMultiset.create(nCopies(count, element)).entrySet().iterator().next();
  }

  public void testToString() {

      queue.add(root);
    }

    @Override
    public boolean hasNext() {
      return !queue.isEmpty();
    }

    @Override
    public T peek() {
      return queue.element();
    }

    @Override
    public T next() {
      T result = queue.remove();
      Iterables.addAll(queue, children(result));
      return result;
    }
  }

      <name>PersistedToolchains</name>
      <superClass>TrackableBase</superClass>
      <description>
        The {@code &lt;toolchains&gt;} element is the root of the descriptor.
        The following table lists all the possible child elements.
      </description>
      <version>1.0.0+</version>
      <fields>
        <field>
          <name>toolchains</name>
          <version>1.0.0+</version>

   * which the elements appear in the given collection.
   *
   * @param elements the elements, all of the same {@code enum} type, that the set should contain
   * @return an immutable set containing those elements, minus duplicates
   */
  public static <E extends Enum<E>> ImmutableSet<E> immutableEnumSet(Iterable<E> elements) {
    if (elements instanceof ImmutableEnumSet) {

    }
  }

  /**
   * A sensible definition of {@link #peek} in terms of {@link #element}. If you override {@link
   * #element}, you may wish to override {@link #peek} to forward to this implementation.
   *
   * @since 7.0
   */
  protected @Nullable E standardPeek() {
    try {
      return element();
    } catch (NoSuchElementException caught) {
      return null;
    }
  }

  /**

    @Override
    public boolean addAll(int index, Collection<? extends V> elements) {
      checkNotNull(elements);
      checkPositionIndex(index, 0);
      throw new IllegalArgumentException("Key does not satisfy predicate: " + key);
    }

    @SuppressWarnings("EmptyList") // ImmutableList doesn't support nullable element types
    @Override
    protected List<V> delegate() {
      return emptyList();
    }

    @Override
    public boolean addAll(int index, Collection<? extends V> elements) {
      checkNotNull(elements);
      checkPositionIndex(index, 0);
      throw new IllegalArgumentException("Key does not satisfy predicate: " + key);
    }

    @SuppressWarnings("EmptyList") // ImmutableList doesn't support nullable element types
    @Override
    protected List<V> delegate() {
      return emptyList();
    }

    StackTraceElement[] elements = e.getStackTrace();
    for (int i = 0; i < elements.length; i++) {
      StackTraceElement element = elements[i];
      if (element.getClassName().equals(clazz) && element.getMethodName().equals(method)) {
        return i;
      }
    }
    throw new AssertionError(
        "Expected element " + clazz + "." + method + " not found in stack trace", e);
  }

     *
     * @param converter
     *            The converter. Must not be {@literal null}.
     * @param propertyNames
     *            The property names to which this converter will be applied. Each element must not be {@literal null} or an empty string.
     * @return A {@link CopyOptions} with the specified converter applied.
     * @see CopyOptions#converter(Converter, CharSequence...)
     */

 * The sources may be Java main classes, test classes, resources or anything else identified by the scope.
 *
 * <h2>Default values</h2>
 * The properties in this interface are defined in the {@code <Source>} element of the
 * {@linkplain org.apache.maven.api.model.Model Maven project descriptor}.
 * For each property, the default value is either empty or documented in the project descriptor.
 */
public interface SourceRoot {
    /**