equations also have a (saddle-node) bifurcation when we
      change the constant torque input, $u_0$.  <!-- This is the subject of
      exercise pend_bifurcation.-->  Finally, let's return to the
      original equations in $\theta$, instead of in $x$.  Only one point to
      make: because of the wrap-around, this system will <em>appear</em> to have
      oscillations.  In fact, the graphical analysis reveals that the pendulum
      will turn forever whenever $|u_0| > mgl$, but now you understand
      t