Top: histogram of residuals obtained by running the free-knot spline fit technique on the observed light curves of HE 0435-1223 (green), and the corresponding synthetic curves (gray). Bottom: distribution of the z_r parameter computed from these residuals. Only one set of observed light curves is available, while the distributions related to the synthetic curves are averaged over 1000 realizations. In this example, the parameters of the generative model have already been adequately adjusted; the synthetic curves shown in Figs. 5 and 6 are drawn from this adjusted model.