code: private void flush() {
          try {
              parent.append(future.get(60, TimeUnit.SECONDS).collect(this));
              parent.append(buffer);
              if (parent instanceof AsyncAppendable) {
                  ((AsyncAppendable) parent).flush();
              }
          } catch (Exception e) {
              throw new MustacheException(
                      MustacheProblem.RENDER_ASYNC_PROCESSING_ERROR, e);
          }
      }
docs: Append the result to the parent.