code: static Continuation<String> stringContinuation(
        WriteContinuation delegate, final StringBuilder buffer) {
      if (delegate.result().isDone()) {
        return done(buffer.toString());
      }
      return new AbstractContinuation<String>(delegate) {
        @Override
        Continuation<String> nextContinuation(WriteContinuation next) {
          return stringContinuation(next, buffer);
        }
      };
    }
docs: Return a string valued continuation. Rendering logic is delegated to the {@link
  WriteContinuation}, but it is assumed that the builder is the render target.