ontroller of the form
      $$u = -k \dot\theta \tilde{E},\quad k>0.$$ I admit that this looks a bit
      strange; it was chosen for the simple reason that it turns the resulting
      "error dynamics" into something simple: $$\dot{\tilde{E}} = - k
      \dot\theta^2 \tilde{E}.$$  Think about the graphical analysis of this
      system if you were to draw $\dot{\tilde{E}}$ vs. $\tilde{E}$ for any fixed
      $\dot\theta$ -- it's a straight line separated from the horizontal axis
      which would imply an 