code: func (as AnnotatedSentence) Phrase(start, end int) (AnnotatedSentence, error) {
  	if start < 0 {
  		return nil, errors.Errorf("Start: %d < 0", start)
  	}
  	if end > len(as) {
  		return nil, errors.Errorf("End: %d > len(as): %d", end, len(as))
  	}
  	return as[start:end], nil
  }
docs: /* Return slices of x */
  // Phrase returns the slice of the sentence. While you can do the same by simply doing as[start:end], this method returns errors instead of panicking