code: func (address HardwareAddr) Mask(mask []byte) []byte {
  	n := len(address)
  	if n != len(mask) {
  		return nil
  	}
  	out := make([]byte, n)
  	for i := 0; i < n; i++ {
  		out[i] = address[i] & mask[i]
  	}
  	return out
  }
docs: // Mask returns the result of masking the address with mask.