- closes#477
- partially reverts 74e6eba
- Note 74e6eba breaks unit tests/behaviour for non-parallel cases.
Automation for parallel cases is not possible since
`tqdm.tqdm._instances` may not be populated in individual threads.
The problem is exhibitable with the modified examples/parallel_bars.py,
the fix is simple check pos is not None rather otherwise will fall into
the trap of 0.