public String toString() {
            return fallback.toString();
          }
        };
    return derive(state.future.catchingAsync(exceptionType, asyncFunction, executor));
  }

  /**
   * Marks this step as the last step in the {@code ClosingFuture} pipeline.
   *
   * <p>The returned {@link Future} is completed when the pipeline's computation completes, or when
   * the pipeline is cancelled.
   *