// closes `doneCh` to indicate to caller to also exit.
	//
	// 2. If caller (Evaluate()) has an error, it sends an error
	// message and waits for this go-routine to quit in
	// FinishWithError()
	//
	// 3. If caller is done, it waits for this go-routine to exit
	// in Finish()

	quitFlag := false
	for !quitFlag {
		select {
		case data := <-writer.errCh: