Institute for Dynamic Systems and Control, ETH-Zurich, Zurich, Switzerland. sherback@idsc.mavt.ethz.ch|Institute for Dynamic Systems and Control, ETH-Zurich, Zurich, Switzerland. sherback@idsc.mavt.ethz.ch|Institute for Dynamic Systems and Control, ETH-Zurich, Zurich, Switzerland. sherback@idsc.mavt.ethz.ch