code: func SubSpan(path, key string, i, j int) (string, error) {
  	si, ok := segIndexByKey(path, key)
  	if !ok {
  		return "", ErrKeySegNotFound
  	}
  
  	if i >= 0 {
  		i++
  	}
  	if j > 0 {
  		j++
  	}
  
  	s, err := Span(path[si:], i, j)
  	if err != nil {
  		return "", err
  	}
  
  	return s, nil
  }
docs: // SubSpan is similar to Span, but only handles the portion of the path
  // subsequent to the provided key. An error is returned if the key cannot be
  // found in the path.