code: func (ipl *IPList) lookup(ip net.IP) (Range, bool) {
  	return lookup(func(i int) net.IP {
  		return ipl.ranges[i].First
  	}, func(i int) Range {
  		return ipl.ranges[i]
  	}, len(ipl.ranges), ip)
  }
docs: // Return the range the given IP is in. Returns nil if no range is found.