code: func (p *Parth) SubSpan(key string, i, j int) string {
  	if p.err != nil {
  		return ""
  	}
  
  	s, err := SubSpan(p.path, key, i, j)
  	p.err = err
  
  	return s
  }
docs: // SubSpan operates the same as the package-level function SubSpan.