code: func (a ARP) IsValid() bool {
  	if len(a) < ARPSize {
  		return false
  	}
  	const htypeEthernet = 1
  	const macSize = 6
  	return a.hardwareAddressSpace() == htypeEthernet &&
  		a.protocolAddressSpace() == uint16(IPv4ProtocolNumber) &&
  		a.hardwareAddressSize() == macSize &&
  		a.protocolAddressSize() == IPv4AddressSize
  }
docs: // IsValid reports whether this is an ARP packet for IPv4 over Ethernet.