}
    }.test();
  }

  @CollectionFeature.Require(value = SUPPORTS_ITERATOR_REMOVE, absent = KNOWN_ORDER)
  public void testRemovingIteratorUnknownOrder() {
    new IteratorTester<E>(
        4,
        MODIFIABLE,
        Arrays.asList(e0(), e1(), e1(), e2()),
        IteratorTester.KnownOrder.UNKNOWN_ORDER) {
      @Override
      protected Iterator<E> newTargetIterator() {

    CharEscaper replacingEscaper =
        new ArrayBasedCharEscaper(SIMPLE_REPLACEMENTS, ' ', '~') {
          private final char[] unknown = new char[] {'?'};

          @Override
          protected char[] escapeUnsafe(char c) {
            return unknown;
          }
        };
    EscaperAsserts.assertBasic(replacingEscaper);

    // Replacements are applied first regardless of whether the character is in

      Helpers.addAll(elements, previousElements);
      Helpers.addAll(elements, Helpers.reverse(nextElements));
      return elements;
    }
  }

  public enum KnownOrder {
    KNOWN_ORDER,
    UNKNOWN_ORDER
  }

  @SuppressWarnings("unchecked") // TODO(cpovirk): Stop using arrays.
  AbstractIteratorTester(
      int steps,
      Iterable<E> elementsToInsertIterable,

    CharEscaper replacingEscaper =
        new ArrayBasedCharEscaper(SIMPLE_REPLACEMENTS, ' ', '~') {
          private final char[] unknown = new char[] {'?'};

          @Override
          protected char[] escapeUnsafe(char c) {
            return unknown;
          }
        };
    EscaperAsserts.assertBasic(replacingEscaper);

    // Replacements are applied first regardless of whether the character is in

        assertThat(network.removeNode(nodeList.get(i))).isTrue();
      }

      assertThat(network.nodes()).hasSize(NUM_NODES - numNodesToRemove);
      // Number of edges remaining is unknown (node's incident edges have been removed).
      AbstractNetworkTest.validateNetwork(network);

      for (int i = numNodesToRemove; i < NUM_NODES; ++i) {
        assertThat(network.removeNode(nodeList.get(i))).isTrue();

    UnicodeEscaper replacingEscaper =
        new ArrayBasedUnicodeEscaper(SIMPLE_REPLACEMENTS, ' ', '~', null) {
          private final char[] unknown = new char[] {'?'};

          @Override
          protected char[] escapeUnsafe(int c) {
            return unknown;
          }
        };
    EscaperAsserts.assertBasic(replacingEscaper);

      Helpers.addAll(elements, previousElements);
      Helpers.addAll(elements, Helpers.reverse(nextElements));
      return elements;
    }
  }

  public enum KnownOrder {
    KNOWN_ORDER,
    UNKNOWN_ORDER
  }

  @SuppressWarnings("unchecked") // TODO(cpovirk): Stop using arrays.
  AbstractIteratorTester(
      int steps,
      Iterable<E> elementsToInsertIterable,

        assertThat(network.removeNode(nodeList.get(i))).isTrue();
      }

      assertThat(network.nodes()).hasSize(NUM_NODES - numNodesToRemove);
      // Number of edges remaining is unknown (node's incident edges have been removed).
      AbstractNetworkTest.validateNetwork(network);

      for (int i = numNodesToRemove; i < NUM_NODES; ++i) {
        assertThat(network.removeNode(nodeList.get(i))).isTrue();

 *
 * <ul>
 *   <li>As a method return type, as an alternative to returning {@code null} to indicate that no
 *       value was available
 *   <li>To distinguish between "unknown" (for example, not present in a map) and "known to have no
 *       value" (present in the map, with value {@code Optional.absent()})
 *   <li>To wrap nullable references for storage in a collection that does not support {@code null}

  }

  @CollectionFeature.Require(absent = KNOWN_ORDER, value = SUPPORTS_ITERATOR_REMOVE)
  public void testIterator_unknownOrderRemoveSupported() {
    runIteratorTest(MODIFIABLE, IteratorTester.KnownOrder.UNKNOWN_ORDER, getSampleElements());
  }

  @CollectionFeature.Require(absent = {KNOWN_ORDER, SUPPORTS_ITERATOR_REMOVE})