code: func GetFreeTCPPorts(n int, offset ...int) (PortList, error) {
  	list := make(PortList, 0, n)
  	start := PortStartingNumber
  	if len(offset) != 0 {
  		start = offset[0]
  	}
  	for i := start; i < start+n; i++ {
  		list = append(list, strconv.Itoa(i))
  	}
  	return list, nil
  }
docs: // GetFreeTCPPorts returns n ports starting from port 20000.