* were obtained by calling {@code snapshot()} on the same {@link PairedStatsAccumulator} without
   * adding any values in between the two calls, or if one is obtained from the other after
   * round-tripping through java serialization. However, floating point rounding errors mean that it
   * may be false for some instances where the statistics are mathematically equal, including

       *   cause if this Future has failed.
       *
       * - And this future *has* failed: This method is called only from handleException (through
       *   getOrInitSeenExceptions). handleException tried to call setException and failed, so
       *   either this Future was cancelled (which we ruled out with the isCancelled check above),

   * Returns the identity function.
   *
   * <p><b>Discouraged:</b> Prefer using a lambda like {@code v -> v}, which is shorter and often
   * more readable.
   */
  // implementation is "fully variant"; E has become a "pass-through" type
  @SuppressWarnings("unchecked")
  public static <E extends @Nullable Object> Function<E, E> identity() {
    return (Function<E, E>) IdentityFunction.INSTANCE;
  }

  // enum singleton pattern

    ReentrantLock lockD = otherFactory.newReentrantLock("LockD");

    // lockA -> lockD
    lockA.lock();
    lockD.lock();
    lockA.unlock();
    lockD.unlock();

    // lockD -> lockA should fail even though lockD is from a different factory.
    lockD.lock();
    PotentialDeadlockException expected =
        assertThrows(PotentialDeadlockException.class, () -> lockA.lock());

    }
    return copy;
  }

  /**
   * Abstracts away the difference between traversing a graph vs. a tree. For a tree, we just take
   * the next element from the next non-empty iterator; for graph, we need to loop through the next
   * non-empty iterator to find first unvisited node.
   */
  private abstract static class Traversal<N> {
    final SuccessorsFunction<N> successorFunction;

   * is called. This will fail if the iterator is infinite and may cause problems if the iterator
   * eagerly fetches data for each source when iterated (rather than producing sources that only
   * load data through their streams). Prefer using the {@link #concat(Iterable)} overload if
   * possible.
   *
   * @param sources the sources to concatenate
   * @return a {@code CharSource} containing the concatenated data

   * LinkedHashMap because one target node may be mapped to both a predecessor and a successor. A
   * LinkedHashMap combines two such edges into a single node-value pair, even though the edges may
   * not have been inserted consecutively.
   */
  @CheckForNull private final List<NodeConnection<N>> orderedNodeConnections;

  private int predecessorCount;
  private int successorCount;

   * initializer.
   */
  private static class LocalClassWithSeeminglyHiddenThisInStaticInitializer {
    static {
      class Local {
        @SuppressWarnings("unused") // through reflection
        Local(LocalClassWithSeeminglyHiddenThisInStaticInitializer outer) {}
      }
      Constructor<?> constructor = Local.class.getDeclaredConstructors()[0];
      int miscalculated = 0;

   * is called. This will fail if the iterator is infinite and may cause problems if the iterator
   * eagerly fetches data for each source when iterated (rather than producing sources that only
   * load data through their streams). Prefer using the {@link #concat(Iterable)} overload if
   * possible.
   *
   * @param sources the sources to concatenate
   * @return a {@code ByteSource} containing the concatenated data

          int entryCount = entry.getCount();
          return countMap.remove(element, entryCount);
        }
        return false;
      }

      /** The hash code is the same as countMap's, though the objects aren't equal. */
      @Override
      public int hashCode() {
        return countMap.hashCode();
      }
    }

    /** We use a special form of unboxing that treats null as zero. */