loop with several parts and conditions

Please take a look at the attached program. The intention is, that you can configure how many pieces of part_a and part_b you want to mill (up to total 9).
Unfortunately if count_part_b is 0, part_b is milled though. In this case I would actually expect elseif in line #53 not to be true and the operations below being skipped, but that's not the case.
Can somebody explain why?! Thanks!

The while loop is cycling trough, until #<position> reached 9. To avoid that, after countdown a+b reached 0, insert this after the position counter. ' #<position> = [#<position> + 1] '

Thanks for your feedback!

In the meantime I found the (a) solution by myself: if I create a new "if" instead of using "elseif" it's working like expected. Still I don't understand why "elseif" is not working. I thought "elseif" and a new "if" has the same effect?!

@tommylight:
Try it with 2 part a and 2 part b! ...and yes, I know that M2 etc is still missing. I wanted to get the loop working first and camotics (which I use for visualising the gcode) is not complaining about it !

@MaHa
Did you try it with a mixture of parts? I don't think it helps....