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.

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