code: def _skew(self, x, z, d=0):
          
          # get the top bound determined by the kurtosis
          kval = (np.tanh(self._poly(z, self._kurtosis_coeffs(d)))+1)/12.
          bdpoly = np.array([
              -1.142468e+04,  3.0939485e+03, -2.0283568e+02,
              -2.1047846e+01, 3.79808487e+00, 1.19679781e-02
          ])
          top = np.polyval(bdpoly, kval)
  
          # limit the skewval to be 0 -> top val
          skew = self._poly(z, self._skew_coeffs(d))
          skewval = top*(np.tanh(skew) + 1) - top
  
          return skewval*(3*x - x**3)
docs: returns the kurtosis parameter for direction d, d=0 is rho, d=1 is z