public <C extends @Nullable Object & @Nullable Closeable> C eventuallyClose(
        @ParametricNullness C closeable, Executor closingExecutor) {
      checkNotNull(closingExecutor);
      if (closeable != null) {
        list.add(closeable, closingExecutor);
      }
      return closeable;
    }
  }

  /**
   * An operation that computes a result.
   *