* returns. This method may be useful when testing the garbage collection mechanism itself, or
   * inhibiting a spontaneous GC initiation in subsequent code.
   *
   * <p>In contrast, a plain call to {@link java.lang.System#gc()} does not ensure finalization
   * processing and may run concurrently, for example, if the JVM flag {@code
   * -XX:+ExplicitGCInvokesConcurrent} is used.
   *

       *
       * (And I don't believe there's any situation in which we call x.combine(y) when x is a plain
       * ImmutableSet.Builder but y is an ImmutableSortedSet.Builder (or vice versa). Certainly
       * ImmutableSortedSet.Builder.combine() is written as if its argument will never be a plain
       * ImmutableSet.Builder: It casts immediately to ImmutableSortedSet.Builder.)
       */
      copyIfNecessary();

    derived.cancel(false);
    assertTrue(primary.isCancelled());
    assertFalse(primary.wasInterrupted());
  }

  // catching() uses a plain Function, so there's no testCatching_resultCancelledAfterFallback().

  // catching() uses a plain Function, so there's no testCatching_nullInsteadOfFuture().

  // Some tests of the exceptionType parameter:

 *       considered "compiled comments."
 *   <li>An explicit {@code if/throw} (as illustrated below) is always acceptable; we still
 *       recommend using our {@link VerifyException} exception type. Throwing a plain {@link
 *       RuntimeException} is frowned upon.
 *   <li>Use of {@link java.util.Objects#requireNonNull(Object)} is generally discouraged, since

     *
     * 1. We'll want to use something like @PolyNull once we can make that work for the various
     * platforms we target.
     *
     * 2. Kotlin's Map type has a getOrDefault method that accepts and returns a "plain V," in
     * contrast to the "V?" type that we're using. As a result, Kotlin sees a conflict between the
     * nullness annotations in ImmutableMap and those in its own Map type. In response, it considers

  /** Support for {@link Iterator#remove()}. */
  SUPPORTS_REMOVE,
  /**
   * Support for {@link ListIterator#add(Object)}; ignored for plain {@link Iterator}
   * implementations.
   */
  SUPPORTS_ADD,
  /**
   * Support for {@link ListIterator#set(Object)}; ignored for plain {@link Iterator}
   * implementations.
   */
  SUPPORTS_SET;

  /**

  /** Support for {@link Iterator#remove()}. */
  SUPPORTS_REMOVE,
  /**
   * Support for {@link ListIterator#add(Object)}; ignored for plain {@link Iterator}
   * implementations.
   */
  SUPPORTS_ADD,
  /**
   * Support for {@link ListIterator#set(Object)}; ignored for plain {@link Iterator}
   * implementations.
   */
  SUPPORTS_SET;

  /**

@GwtCompatible
@ElementTypesAreNonnullByDefault
final class NullnessCasts {
  /**
   * Accepts a {@code @Nullable T} and returns a plain {@code T}, without performing any check that
   * that conversion is safe.
   *
   * <p>This method is intended to help with usages of type parameters that have {@linkplain

    derived.cancel(false);
    assertTrue(primary.isCancelled());
    assertFalse(primary.wasInterrupted());
  }

  // catching() uses a plain Function, so there's no testCatching_resultCancelledAfterFallback().

  // catching() uses a plain Function, so there's no testCatching_nullInsteadOfFuture().

  // Some tests of the exceptionType parameter:

    }
  }

  /**
   * Returns a {@link Collection} of all the permutations of the specified {@link Collection}.
   *
   * <p><i>Notes:</i> This is an implementation of the Plain Changes algorithm for permutations
   * generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
   * Section 7.2.1.2.
   *