code: func addrUnique(inuse map[string]string, name string, addr net.Addr) error {
  	key := addr.Network() + ":" + addr.String()
  	if other, ok := inuse[key]; ok {
  		return fmt.Errorf("%s address %s already configured for %s", name, addr.String(), other)
  	}
  	inuse[key] = name
  	return nil
  }
docs: // addrUnique checks if the given address is already in use for another
  // protocol.