* {@link CloseableList} may not be subsumed.
     */
    CLOSING,

    /** The {@link CloseableList} has been closed. It may not be further subsumed. */
    CLOSED,

    /**
     * {@link ClosingFuture#finishToValueAndCloser(ValueAndCloserConsumer, Executor)} has been
     * called. The step may not be further subsumed, nor may {@link #finishToFuture()} be called.
     */