@Override
        protected List<LockGraphNode> initialValue() {
          return newArrayListWithCapacity(3);
        }
      };

  /**
   * A Throwable used to record a stack trace that illustrates an example of a specific lock
   * acquisition ordering. The top of the stack trace is truncated such that it starts with the
   * acquisition of the lock in question, e.g.
   *
   * <pre>

    Iterator<String> itr2 = Iterators.concat(itr1, singletonIterator("d"));
    assertThat(itr2.next()).isEqualTo("c");
    assertThat(itr2.next()).isEqualTo("d");
  }

  /** Illustrates the somewhat bizarre behavior when a null is passed in. */
  public void testConcatContainingNull() {
    Iterator<Iterator<Integer>> input =
        (Iterator<Iterator<Integer>>)

    Iterator<String> itr2 = Iterators.concat(itr1, singletonIterator("d"));
    assertThat(itr2.next()).isEqualTo("c");
    assertThat(itr2.next()).isEqualTo("d");
  }

  /** Illustrates the somewhat bizarre behavior when a null is passed in. */
  public void testConcatContainingNull() {
    Iterator<Iterator<Integer>> input =
        (Iterator<Iterator<Integer>>)