code: func (p *Pinger) AddIP(ipaddr string) error {
  	addr := net.ParseIP(ipaddr)
  	if addr == nil {
  		return fmt.Errorf("%s is not a valid textual representation of an IP address", ipaddr)
  	}
  	p.mu.Lock()
  	p.addrs[addr.String()] = &net.IPAddr{IP: addr}
  	if isIPv4(addr) {
  		p.hasIPv4 = true
  	} else if isIPv6(addr) {
  		p.hasIPv6 = true
  	}
  	p.mu.Unlock()
  	return nil
  }
docs: // AddIP adds an IP address to Pinger. ipaddr arg should be a string like
  // "192.0.2.1".