code: def blockstack_tx_filter( tx ):
      
      if not 'nulldata' in tx:
          return False
      
      if tx['nulldata'] is None:
          return False
  
      payload = binascii.unhexlify( tx['nulldata'] )
      if payload.startswith(blockstack_magic_bytes()):
          return True
  
      else:
          return False
docs: Virtualchain tx filter function:
      * only take txs whose OP_RETURN payload starts with 'id'