> point to slot 1
[
   - initialize slot 1 to zero
]
>> point to slot 3
[
   - set slot 3 to zero
]
+ set slot 3 to 1
> point to slot 4
[
   - initialize slot 4 to 0
]
<<<< point to X
[ while X
   >>>>> point to Z
   [
      - set Z to 0
   ]
   <<< point to Y
   [ if Y is nonzero, set Z to 1
      - decrement Y
      >>> point to Z
      + Z = 1
      < point to slot 4 which is always zero
   ] loop ends after one iteration
   < point to either slot 1 or slot 3
   [ 
      if we are here then we are pointing to slot 3
      << point to slot 1
   }
   < point to X
   - decrement X
]
>>>>> point to Z
[
   Y >= X, do whatever here
   < point to slot 4, always zero to terminate the loop
]