Before revision:   Using the Selection problem as an example , we show that it is possible to produce instances which are ten times as hard to solve on average for a state-of-the-art mixed-integer programming solver  .
After revision:  Using the Selection and Traveling Salesman problems as examples , we show that it is possible to produce instances which are ten times as hard to solve on average for a state-of-the-art mixed-integer programming solver .