code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          if self._new:
              if nu.fabs(z) < 10.**-6.:
                  y= 0.5*self._alpha*R
                  return nu.pi*self._alpha*(special.i0(y)*special.k0(y)-special.i1(y)*special.k1(y)) \
                      +nu.pi/4.*self._alpha**2.*R*(special.i1(y)*(3.*special.k0(y)+special.kn(2,y))-special.k1(y)*(3.*special.i0(y)+special.iv(2,y)))
              raise AttributeError("'R2deriv' for RazorThinExponentialDisk not implemented for z =/= 0")
docs: NAME:
             R2deriv
          PURPOSE:
             evaluate R2 derivative
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             -d K_R (R,z) d R
          HISTORY:
             2012-12-27 - Written - Bovy (IAS)