checkArgument(((BaseGraph<?>) tree).isDirected(), "Undirected graphs can never be trees.");
    }
    if (tree instanceof Network) {
      checkArgument(((Network<?, ?>) tree).isDirected(), "Undirected networks can never be trees.");
    }
    return new Traverser<N>(tree) {
      @Override
      Traversal<N> newTraversal() {
        return Traversal.inTree(tree);
      }
    };
  }

  /**

  private static Map<String, String> toHashMap(Entry<String, String>[] entries) {
    return populate(new HashMap<String, String>(), entries);
  }

  // TODO: call conversion constructors or factory methods instead of using
  // populate() on an empty map
  private static <T, M extends Map<T, String>> M populate(M map, Entry<T, String>[] entries) {
    for (Entry<T, String> entry : entries) {
      map.put(entry.getKey(), entry.getValue());
    }

        return ImmutableBiMap.copyOf(sourceMap);
      }
    },
    COPY_OF_ENTRIES {
      @Override
      public ImmutableBiMap<Object, Object> create(List<Entry<?, ?>> entries) {
        return ImmutableBiMap.copyOf(entries);
      }
    },
    BUILDER_PUT_ONE_BY_ONE {
      @Override
      public ImmutableBiMap<Object, Object> create(List<Entry<?, ?>> entries) {
        ImmutableBiMap.Builder<Object, Object> builder = ImmutableBiMap.builder();

      Iterable<? extends Entry<? extends K, ? extends V>> entries) {
    int estimatedSize =
        (entries instanceof Collection)
            ? ((Collection<?>) entries).size()
            : ImmutableCollection.Builder.DEFAULT_INITIAL_CAPACITY;
    return new Builder<K, V>(estimatedSize).putAll(entries).build();
  }

  ImmutableBiMap() {}

  /**
   * {@inheritDoc}
   *

  int getHash();

  /** Returns the key for this entry. */
  @CheckForNull
  K getKey();

  /*
   * Used by entries that use access order. Access entries are maintained in a doubly-linked list.
   * New entries are added at the tail of the list at write time; stale entries are expired from
   * the head of the list.
   */

  /** Returns the time that this entry was last accessed, in ns. */

          entryIndex = next - 1;
          entry = entries[entryIndex];
          next = CompactHashing.getNext(entry, mask);
        } while (next != srcNext);
        // here, entries[entryIndex] points to the old entry location; update it
        entries[entryIndex] = CompactHashing.maskCombine(entry, dstIndex + 1, mask);
      }
    } else {
      elements[dstIndex] = null;
      entries[dstIndex] = 0;
    }
  }

  }

  private final transient ImmutableList<Entry<K, V>> entries;
  private final Map<K, V> forwardDelegate;
  private final Map<V, K> backwardDelegate;

  private JdkBackedImmutableBiMap(
      ImmutableList<Entry<K, V>> entries, Map<K, V> forwardDelegate, Map<V, K> backwardDelegate) {
    this.entries = entries;
    this.forwardDelegate = forwardDelegate;
    this.backwardDelegate = backwardDelegate;
  }

    Set<Entry<@Nullable String, @Nullable Boolean>> entries = map.entrySet();
    assertTrue(entries.contains(Maps.immutableEntry("a", true)));
    assertTrue(entries.contains(Maps.<String, @Nullable Boolean>immutableEntry("b", null)));
    assertTrue(
        entries.contains(Maps.<@Nullable String, @Nullable Boolean>immutableEntry(null, null)));

    assertFalse(entries.contains(Maps.<String, @Nullable Boolean>immutableEntry("c", null)));

    return getEntryNullReplaces().getKey();
  }

  private Entry<K, V> getEntryNullReplaces() {
    Iterator<Entry<K, V>> entries = getSampleElements().iterator();
    for (int i = 0; i < getNullLocation(); i++) {
      entries.next();
    }
    return entries.next();
  }

  /** @return an array of the proper size with {@code null} as the value of the middle element. */

    super(delegate);
  }

  RegularImmutableMap(Entry<? extends K, ? extends V>... entries) {
    this(/* throwIfDuplicateKeys= */ true, entries);
  }

  RegularImmutableMap(boolean throwIfDuplicateKeys, Entry<? extends K, ? extends V>[] entries) {
    super(throwIfDuplicateKeys, entries);
  }