The code between REPEAT and UNTIL keywords will be processed until the
condition is false, if the condition is true, it will terminate. It is
also possible to use EXITIF keyword for early
termination. If DO keyword is used, commands will
be processed while terminating the loop. If IS
keyword is used, the loop can return a list of values.
UNTIL <exp> [ DO <statements> ] [ IS <list> ]
If you add 'N' after UNTIL, the result of <exp> will be negated: