code: def dre_dm(self, pars):
          r
          self._set_parameters(pars)
          numerator = -self.otc * (np.cos(self.ang) + self.otc)
          result = numerator / self.denom
          result *= self.rho0
          return result
docs: r"""
          :math:`\frac{\partial \hat{\rho'}(\omega)}{\partial m} = - \rho_0 m
          (\omega \tau)^c \frac{(cos(\frac{c \pi}{2}) + (\omega \tau)^c)}{1 + 2
          (\omega \tau)^c cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}}`