code: func (a *IPRangeAllocator) Subtract(iprange *IPRange) {
  	curBig := big.NewInt(0)
  	curBig.SetBytes(iprange.Start)
  	endBig := big.NewInt(0)
  	endBig.SetBytes(iprange.End)
  
  	for ; curBig.Cmp(endBig) < 1; curBig = curBig.Add(big.NewInt(1), curBig) {
  		ip := a.bigIntToIP(curBig)
  		if a.ipRange.Contains(ip) {
  			a.Reserve(ip)
  		}
  	}
  }
docs: // Subtract marks all of the IPs from another IPRange as reserved in the current
  // allocator.