* consistency, and because the return type conveys the immutability guarantee.
   *
   * <p><b>Performance note:</b> the instance returned is a singleton.
   */
  @SuppressWarnings({"unchecked"}) // fully variant implementation (never actually produces any Es)
  public static <E> ImmutableSet<E> of() {
    return (ImmutableSet<E>) RegularImmutableSet.EMPTY;
  }

  /**

import static com.google.common.math.StatsTesting.LARGE_INTEGER_VALUES_POPULATION_VARIANCE;
import static com.google.common.math.StatsTesting.LARGE_INTEGER_VALUES_STATS;
import static com.google.common.math.StatsTesting.LARGE_LONG_VALUES_MEAN;
import static com.google.common.math.StatsTesting.LARGE_LONG_VALUES_POPULATION_VARIANCE;
import static com.google.common.math.StatsTesting.LARGE_LONG_VALUES_STATS;

   * already been re-added.
   */
  // Technically should accept RemovalNotification<? extends K, ? extends V>, but because
  // RemovalNotification is guaranteed covariant, let's make users' lives simpler.
  void onRemoval(RemovalNotification<K, V> notification);

      if (values.hasAnyNonFinite()) {
        assertWithMessage("population variance of " + values).that(populationVariance).isNaN();
        assertWithMessage("population variance by addAll(Stats) of " + values)
            .that(populationVarianceByAddAllStats)
            .isNaN();
      } else {
        assertWithMessage("population variance of " + values)
            .that(populationVariance)
            .isWithin(ALLOWED_ERROR)

      BigInteger.valueOf(Integer.MAX_VALUE)
          .multiply(BigInteger.valueOf(3L))
          .divide(BigInteger.valueOf(4L))
          .doubleValue();
  static final double LARGE_INTEGER_VALUES_POPULATION_VARIANCE =
      BigInteger.valueOf(Integer.MAX_VALUE)
          .multiply(BigInteger.valueOf(Integer.MAX_VALUE))
          .divide(BigInteger.valueOf(16L))
          .doubleValue();

      }

      @Override
      boolean hashFloodingDetected(Object[] hashTable) {
        int maxRunBeforeFallback = maxRunBeforeFallback(hashTable.length);
        int mask = hashTable.length - 1;

        // Invariant: all elements at indices in [knownRunStart, knownRunEnd) are nonnull.
        // If knownRunStart == knownRunEnd, this is vacuously true.
        // When knownRunEnd exceeds hashTable.length, it "wraps", detecting runs around the end

  public static <E> ImmutableSet<E> copyOf(Iterable<? extends E> elements) {
    if (elements instanceof ImmutableSet && !(elements instanceof ImmutableSortedSet)) {
      @SuppressWarnings("unchecked") // all supported methods are covariant
      ImmutableSet<E> set = (ImmutableSet<E>) elements;
      return set;
    }
    return copyOf(elements.iterator());
  }

  public static <E> ImmutableSet<E> copyOf(Iterator<? extends E> elements) {

    FluentIterable<? extends Number> numbers = getSomeNumbers();

    // Sadly, the following is what users will have to do in some circumstances.

    @SuppressWarnings("unchecked") // safe covariant cast
    Optional<Number> first = (Optional<Number>) numbers.first();
    Number value = first.or(0.5); // fine
  }

  @J2ktIncompatible
  @GwtIncompatible // NullPointerTester

 * href="https://github.com/google/guava/wiki/UsingAndAvoidingNullExplained#optional">using {@code
 * Optional}</a>.
 *
 * @param <T> the type of instance that can be contained. {@code Optional} is naturally covariant on
 *     this type, so it is safe to cast an {@code Optional<T>} to {@code Optional<S>} for any
 *     supertype {@code S} of {@code T}.
 * @author Kurt Alfred Kluever
 * @author Kevin Bourrillion
 * @since 10.0
 */

    // Count how long it actually took to return; we'll accept any number between the expected delay
    // and the approximate actual delay, to be robust to variance in thread scheduling.
    char overWaitNanosFirstDigit =
        Long.toString(
                thread.timeSpentBlocked - TimeUnit.MILLISECONDS.toNanos(longWait ? 5000 : 3000))
            .charAt(0);