}
    };
  }

  /** Returns the children of the specified node. Must not contain null. */
  public abstract Iterable<T> children(T root);

  /**
   * Returns an unmodifiable iterable over the nodes in a tree structure, using pre-order traversal.
   * That is, each node's subtrees are traversed after the node itself is returned.
   *

import static com.google.common.collect.ReflectionFreeAssertThrows.assertThrows;
import static com.google.common.collect.testing.IteratorFeature.MODIFIABLE;
import static com.google.common.collect.testing.IteratorFeature.UNMODIFIABLE;
import static com.google.common.truth.Truth.assertThat;
import static java.util.Arrays.asList;
import static java.util.Collections.emptyList;

     *
     * @return an unmodifiable map of user properties
     */
    @Nonnull
    Map<String, String> userProperties();

    /**
     * Returns a map of system properties for the Maven execution.
     * These include both Java system properties and Maven-specific system properties.
     *
     * @return an unmodifiable map of system properties
     */
    @Nonnull

   * interfaces.
   */
  public static final Set<IteratorFeature> UNMODIFIABLE = emptySet();

  /**
   * A set containing all of the optional features of the {@link Iterator} and {@link ListIterator}
   * interfaces.
   */
  public static final Set<IteratorFeature> MODIFIABLE =
      unmodifiableSet(new LinkedHashSet<>(asList(values())));

                                                        + cls.getName()
                                                        + "; it is an unmodifiable collection.");
                                            }
                                            continue;
                                        }

   *
   * <p>The returned table will be serializable if the specified table is serializable.
   *
   * @param table the row-sorted table for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified table
   * @since 11.0
   */
  public static <R extends @Nullable Object, C extends @Nullable Object, V extends @Nullable Object>

import static com.google.common.collect.Lists.transform;
import static com.google.common.collect.ReflectionFreeAssertThrows.assertThrows;
import static com.google.common.collect.testing.IteratorFeature.UNMODIFIABLE;
import static com.google.common.truth.Truth.assertThat;
import static java.lang.System.arraycopy;
import static java.util.Arrays.asList;
import static java.util.Collections.emptyList;

  private Iterables() {}

  /** Returns an unmodifiable view of {@code iterable}. */
  public static <T extends @Nullable Object> Iterable<T> unmodifiableIterable(
      Iterable<? extends T> iterable) {
    checkNotNull(iterable);
    if (iterable instanceof UnmodifiableIterable || iterable instanceof ImmutableCollection) {
      @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe

   * serializable.
   *
   * <p><b>Java 8+ users and later:</b> Prefer {@link Collections#unmodifiableNavigableSet}.
   *
   * @param set the navigable set for which an unmodifiable view is to be returned
   * @return an unmodifiable view of the specified navigable set
   * @since 12.0
   */
  public static <E extends @Nullable Object> NavigableSet<E> unmodifiableNavigableSet(
      NavigableSet<E> set) {

     * Each result contains the original request, the response (if successful), and any error
     * that occurred during processing.
     *
     * @return An unmodifiable list of request results
     */
    public List<RequestResult<?, ?>> getResults() {
        return results;
    }