// this has 2 benefits
              // 1. for long chains of futures strung together with setFuture we consume less stack
              // 2. we avoid allocating Cancellation objects at every level of the cancellation
              //    chain
              // We can only do this for TrustedFuture, because TrustedFuture.cancel is final and

     *
     * <p>This method exists so that the {@link Segment} code can be generic in terms of {@code S},
     * the type of the concrete implementation.
     */
    abstract S self();

    /** Drains the reference queues used by this segment, if any. */
    @GuardedBy("this")
    void maybeDrainReferenceQueues() {}

    /** Clears the reference queues used by this segment, if any. */

 * article on <a href="https://github.com/google/guava/wiki/ListenableFutureExplained">{@code
 * ListenableFuture}</a>.
 *
 * <p>The main purpose of {@code ListenableFuture} is to help you chain together a graph of
 * asynchronous operations. You can chain them together manually with calls to methods like {@link
 * Futures#transform(ListenableFuture, Function, Executor) Futures.transform}, but you will often