fwr_sv(beta, m)
$$f(m,\bm{\beta}) = \beta_0+\beta_1\exp\left(-\frac{m}{\tau_1}\right)+\beta_2\left[\left(\frac{m}{\tau_1}\right)\exp\left(-\frac{m}{\tau_1}\right)\right] +\beta_3\left[\left(\frac{m}{\tau_2}\right)\exp\left(-\frac{m}{\tau_2}\right)\right].$$