* eliminate duplicate keys (for buildKeepingLast()) then we have to ensure that a later call to
   * buildOrThrow() will still throw as if the duplicates had not been eliminated. And the exception
   * message must mention two values that were associated with the duplicate key in two different
   * calls to Builder.put (though we don't really care *which* two values if there were more than

   *
   * However, for a variety of reasons, Google developers have written a ton of code over the past
   * decade that assumes that they can use checkNotNull for non-precondition checks. I had hoped to
   * take a principled stand on this, but the amount of such code is simply overwhelming. To avoid
   * creating a lot of compile errors that users would not find to be informative, we're giving in

    final Object localValue = value;
    return localValue instanceof Cancellation;
  }

  /**
   * {@inheritDoc}
   *
   * <p>If a cancellation attempt succeeds on a {@code Future} that had previously been {@linkplain
   * #setFuture set asynchronously}, then the cancellation will also be propagated to the delegate
   * {@code Future} that was supplied in the {@code setFuture} call.
   *

   * Java Language Specification. Tasks execute with the same happens-before order that the function
   * calls to {@link Executor#execute `execute()`} that submitted those tasks had.
   *
   * <p>The executor uses {@code delegate} in order to {@link Executor#execute execute} each task in
   * turn, and does not create any threads of its own.
   *

    K getKey();

    /** Gets the value for the entry. */
    V getValue();
  }

  /*
   * Note: the following classes have a lot of duplicate code. It sucks, but it saves a lot of
   * memory. If only Java had mixins!
   */

  /** Base class for {@link InternalEntry} implementations for strong keys. */
  abstract static class AbstractStrongKeyEntry<K, V, E extends InternalEntry<K, V, E>>
      implements InternalEntry<K, V, E> {

        return whenClosed = new CountDownLatch(1);
      }
    }
  }

  /**
   * Returns an object that can be used to wait until this objects' deferred closeables have all had
   * {@link Runnable}s that close them submitted to each one's closing {@link Executor}.
   */
  @VisibleForTesting
  CountDownLatch whenClosedCountDown() {
    return closeables.whenClosedCountDown();
  }