code: func (p *parser) parseSelectorGroup() (result Selector, err error) {
  	result, err = p.parseSelector()
  	if err != nil {
  		return
  	}
  
  	for p.i < len(p.s) {
  		if p.s[p.i] != ',' {
  			return result, nil
  		}
  		p.i++
  		c, err := p.parseSelector()
  		if err != nil {
  			return nil, err
  		}
  		result = unionSelector(result, c)
  	}
  
  	return
  }
docs: // parseSelectorGroup parses a group of selectors, separated by commas.