10.1.4 UNTIL Modifier
  The UNTIL modifier, like the FOR modifier, creates a
  single-line loop. However, instead of using a formal loop
  variable, you specify the terminating condition with a condi-
  tional expression. The modified statement executes repeatedly
  as long as the condition is false. For example:
  B = B + 1 UNTIL (A - B) < 0.0001