this.successorFunction = checkNotNull(successorFunction);
  }

  /**
   * Creates a new traverser for the given general {@code graph}.
   *
   * <p>Traversers created using this method are guaranteed to visit each node reachable from the
   * start node(s) at most once.
   *
   * <p>If you know that no node in {@code graph} is reachable by more than one path from the start

   * is now at some position before {@code index}. The second element is the one that was swapped
   * down to replace the element at {@code index}. This fact is used by iterator.remove so as to
   * visit elements during a traversal once and only once.
   */
  @VisibleForTesting
  @CanIgnoreReturnValue
  @CheckForNull
  MoveDesc<E> removeAt(int index) {
    checkPositionIndex(index, size);
    modCount++;

 * new TypeVisitor() {
 *   protected void visitParameterizedType(ParameterizedType t) {
 *     visit(t.getOwnerType());
 *     visit(t.getActualTypeArguments());
 *   }
 *   protected void visitGenericArrayType(GenericArrayType t) {
 *     visit(t.getGenericComponentType());
 *   }
 *   protected void visitTypeVariable(TypeVariable<?> t) {
 *     throw new IllegalArgumentException("Cannot contain type variable.");
 *   }

question.  `What sort of people live about here?'

  `In THAT direction,' the Cat said, waving its right paw round,
`lives a Hatter:  and in THAT direction,' waving the other paw,
`lives a March Hare.  Visit either you like:  they're both mad.'

  `But I don't want to go among mad people,' Alice remarked.

  `Oh, you can't help that,' said the Cat:  `we're all mad here.
I'm mad.  You're mad.'

    int nTasks = 1000;
    int nSeededEntries = 100;
    Set<String> expectedKeys = Sets.newHashSetWithExpectedSize(nTasks + nSeededEntries);
    // seed the map, so its segments have a count>0; otherwise, clear() won't visit the in-progress
    // entries
    for (int i = 0; i < nSeededEntries; i++) {
      String s = "b" + i;
      cache.getUnchecked(s);
      expectedKeys.add(s);
    }
    computationShouldWait.set(true);