code: def dim_dm(self, pars):
          r
          self._set_parameters(pars)
          numerator = -self.otc * np.sin(self.ang)
          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{sin(\frac{c \pi}{2})}{1 + 2 (\omega \tau)^c
          cos(\frac{c \pi}{2}) + (\omega \tau)^{2 c}}`