DO

   Synopsis:
      Initiates a DO..UNTIL loop

   Notes:
      Every DO statement must have a corresponding UNTIL statement.
      The loop continues until the condition is evaluated as 'false'.
      Unlike a WHILE..LOOP loop, a DO..UNTIL loop will be executed at least
         once.
      Following an EXIT DO statement, the loop terminates immediately. Execution
         resumes after the next UNTIL statement.

   Availability:
      DO is not available in scripts with primitive line numbers.

   Examples:
      ! Print the numbers 1 to 10 in sequence
      LET count = 0
      DO
         LET count = count + 1
         PRINT count
      UNTIL count >= 10

      ! Keep rolling the dice until we get a 3 or a 6
      DO
         LET dice = Int (Rnd (6) + 1)
         PRINT dice
         IF dice = 3 THEN EXIT DO
         PRINT "Phew! We didn't roll a 3"
      UNTIL dice = 6

      ! This loop runs forever
      DO
         PRINT "Hello, world!"
      UNTIL 0

      ! This loop runs exactly once
      DO
         PRINT "Hello, world!"
      UNTIL 1   
