code: func (as AnnotatedSentence) String() string {
  	var buf bytes.Buffer
  	for i, a := range as {
  		buf.WriteString(fmt.Sprintf("%s/%s", a.Value, a.POSTag))
  		if i < len(as)-1 {
  			buf.WriteString(" ")
  		}
  	}
  	return buf.String()
  }
docs: // Stringer interface