Who owes whom?

Previously a list of expenses could have two states: open or closed. Now it has three states: open, to paid, paid. When a list is closed, it becomes "to pay" until all the induced debts are mark as paid. Once a list is "paid", it can become "to pay" if one the debts induced by this list (or one debt simplifying the debts induced by this list) is marked as "not paid". And it becomes "open" again if you re-open the list.