code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          m= 4.*R*self.a/((R+self.a)**2+z**2)
          return -4.*self.a/nu.sqrt((R+self.a)**2+z**2)*special.ellipk(m)
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2018-08-04 - Written - Bovy (UofT)