return (result == null) ? keySet = createKeySet() : result;
  }

  /*
   * This could have a good default implementation of return new ImmutableKeySet<K, V>(this),
   * but ProGuard can't figure out how to eliminate that default when RegularImmutableMap
   * overrides it.
   */
  abstract ImmutableSet<K> createKeySet();

  UnmodifiableIterator<K> keyIterator() {

    assertEquals("b", consumingIterator.next());
    assertThat(list).isEmpty();

    assertFalse(consumingIterator.hasNext());
  }

  @GwtIncompatible // ?
  // TODO: Figure out why this is failing in GWT.
  public void testConsumingIterator_duelingIterators() {
    // Test data
    List<String> list = Lists.newArrayList("a", "b");

    // Test & Verify

                     pass -XepCompilingTestOnlyCode, and that argument needs to
                     be passed as part of the same <arg> as -Xplugin:ErrorProne,
                     and I gave up trying to figure out how to do that for test
                     compilation only. -->
                <arg>-Xplugin:ErrorProne -Xep:NullArgumentForNonNullParameter:OFF -Xep:Java8ApiChecker:ERROR</arg>

      // We're looking for the smallest charsPerChunk such that bitsPerChar * charsPerChunk is a
      // multiple of 8. A multiple of 8 has 3 low zero bits, so we just need to figure out how many
      // extra zero bits we need to add to the end of bitsPerChar to get 3 in total.
      // The logic here would be wrong for bitsPerChar > 8, but since we require distinct ASCII

      default:
        throw new AssertionError();
    }
  }

  @GwtIncompatible // TODO
  private static BigInteger sqrtFloor(BigInteger x) {
    /*
     * Adapted from Hacker's Delight, Figure 11-1.
     *
     * Using DoubleUtils.bigToDouble, getting a double approximation of x is extremely fast, and
     * then we can get a double approximation of the square root. Then, we iteratively improve this

    Iterators.addAll(list, elements);
    return list;
  }

  @VisibleForTesting
  static int computeArrayListCapacity(int arraySize) {
    checkNonnegative(arraySize, "arraySize");

    // TODO(kevinb): Figure out the right behavior, and document it
    return Ints.saturatedCast(5L + arraySize + (arraySize / 10));
  }

  /**
   * Creates an {@code ArrayList} instance backed by an array with the specified initial size;

  }

  /**
   * Returns the {@link Method} instance for {@link #testAdd_unsupportedNotPresent()} so that tests
   * can suppress it with {@code FeatureSpecificTestSuiteBuilder.suppressing()} while we figure out
   * what to do with <a href="http://goo.gl/qJBruX">{@code ConcurrentHashMap} support for {@code
   * entrySet().add()}</a>.
   */
  @J2ktIncompatible
  @GwtIncompatible // reflection

    assertEquals("b", consumingIterator.next());
    assertThat(list).isEmpty();

    assertFalse(consumingIterator.hasNext());
  }

  @GwtIncompatible // ?
  // TODO: Figure out why this is failing in GWT.
  public void testConsumingIterable_duelingIterators() {
    // Test data
    List<String> list = Lists.newArrayList(asList("a", "b"));

    // Test & Verify

          }

          @Override
          public void onFailure(Throwable t) {
            failureCalls[0]++;
          }
        };
    addCallback(future, callback, directExecutor());
    future.set(result);
    assertEquals(result, future.get());
    assertThat(successCalls[0]).isEqualTo(1);
    assertThat(failureCalls[0]).isEqualTo(0);
  }

  public void testOnSuccessThrowsError() throws Exception {

 *     Guava 27.0
 */
public final class InternalFutures {
  /**
   * Usually returns {@code null} but, if the given {@code Future} has failed, may <i>optionally</i>
   * return the cause of the failure. "Failure" means specifically "completed with an exception"; it
   * does not include "was cancelled." To be explicit: If this method returns a non-null value,
   * then:
   *
   * <ul>