if (isCancelled()) {
      builder.append("CANCELLED");
    } else if (isDone()) {
      addDoneString(builder);
    } else {
      addPendingString(builder); // delegates to addDoneString if future completes midway
    }
    return builder.append("]").toString();
  }

  /**
   * Provide a human-readable explanation of why this future has not yet completed.
   *