I revisited the Introduction to Bitshares video on youtube today, and I realized that I hadn't fully understood how these bitAssets are created and destroyed. I understand that the bitUSD is created when you have a long and short position that can agree to the price of the bitUSD/bts. So now we have a bitUSD which is collateralized by the short position, but what happens when the person shorting covers his/her position? It would make sense to me that the short position repurchases the bitUSD on the open market thereby switching his/her obligation to collateralizing the bitUSD he originally created. In this instance the second seller of the bitUSD is not creating new bitUSD, but instead rebacking old bitUSD. Assuming the margin calls are conducted in the same way where the bitUSD is still purchased on the open market I don't see how any bitUSD is ever destroyed. Can someone please explain to me if and how bitUSD is destroyed?