DO       RF, offset
         R1, R2, direction-1, offset

F/DO     ...
         ...

NEXT     RF
         R1, R2, direction-1

F/NEXT   ...
         ...

LEAVE    RF, offset
         RF, level, offset

F/LEAVE  ...
         ...         

ITERATE  RF
         R1, R2, direction-1

F/ITERATE...
         ...                  

