docs: def db_check( block_id, opcode, op, txid, vtxindex, checked, db_state=None ):
     """
     Given the block ID and a parsed operation, check to see if this is a *valid* operation
     for the purposes of this virtual chain's database.
     
     Return True if so; False if not.
     """
     print "\nreference implementation of db_check\n"
     return False