code: func ParsePortSet(s string) (*PortSet, error) {
  	ps := &PortSet{}
  
  	if s == "" {
  		return nil, errors.New("must specify at least one port")
  	}
  
  	ranges := strings.Split(s, ",")
  
  	for _, r := range ranges {
  		portRange, err := ParsePortRange(r)
  
  		if err != nil {
  			return nil, err
  		}
  
  		*ps = append(*ps, *portRange)
  	}
  
  	return ps, nil
  }
docs: // ParsePortSet parses the s to a PortSet.
  // The s shoud be a comma separated string.