Dear SamT,

Thanks a lot for your answer. It's a bit trickier with the liability though. It does incur only in 6 months, BUT, if, for example, you have taken 2 pieces less in the 3rd month (March) and then taken 2 pieces more in the 4th month (April), in October (March +6 months) you don't have to take an extra 2 pieces you did not take in March, because you already have taken 2 more in April. It's like having an imaginary stock, where you can put things and take them out again. Do you have an idea how to make that? This is the point where I get lost..

Thanks a lot for your help again!