Add models and proofs for paper
  Yanni Kouskoulas, David Renshaw, André Platzer and Peter Kazanzides.
  Certifying the safe design of a virtual fixture control algorithm for a surgical robot.
  In Calin Belta and Franjo Ivancic, editors, Hybrid Systems: Computation and Control (part of CPS Week 2013), HSCC'13, Philadelphia, PA, USA, April 8-13, 2013, ACM, 2013.