def previous_block_id(self):
        return b2h_rev(self.previous_block_hash)
We need to check if the domain is the root domain in a zone.
dataset/ETHPy150Open maraoz/proofofexistence/pycoin/block.py/BlockHeader.previous_block_id--crossover with--dataset/ETHPy150Open rtucker-mozilla/mozilla_inventory/mozdns/cname/models.py/CNAME.check_SOA_condition