code: def _evaluate(self,z,t=0.):
          
          return self._Pot(self._R,z,phi=self._phi,t=t,use_physical=False)\
              -self._Pot(self._R,0.,phi=self._phi,t=t,use_physical=False)
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential
          INPUT:
             z
             t
          OUTPUT:
            Pot(z,t;R)
          HISTORY:
             2010-07-13 - Written - Bovy (NYU)