1. Workforce Management, INFORM Software GmbH, D-52076 Aachen, Germany;
2. Lehrstuhl für Operations Research, RWTH Aachen University, D-52072 Aachen, Germany;
3. Institut für Mathematik, Technische Universität Berlin, D-10623 Berlin, Germany;
4. Beijing Institute for Scientific and Engineering Computing, Beijing University of Technology, Beijing 100124, China