<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate(self):
 """Assure this is a valid VLAN header instance."""

        if self.tpid.value not in (EtherType.VLAN, EtherType.VLAN_QINQ):
            raise UnpackException
        return