[see discussion in @2017ApJ...836...12K and Sect. \[sec:models\] below], so the use of a Gaussian approximation is further justified. For stronger lines, like the Mg [ii]{} 2791.6Å, we follow the original work of @1984SoPh...93..105I in assuming that the flare emission originates mostly from a separate, optically thin slab of (moving) plasma overlaying the stationary atmosphere. In Figs. \[fig:spectraA\] and \[fig:spectraB\], the quasi-stationary component of each spectral line is shown with black dotted lines, while the red-shifted component is displayed in red. The final overall fit is given as a thin black line. We constrain the central component in all lines to remain within $\pm$ 2 km/s of the rest wavelength, allowing the second shifted component to return as far as possible towards the line rest wavelength, and track its complete velocity evolution. At times, the spectral range in proximity of Fe [ii]{} 2813.3 Å can be subject to some ambiguities due to the presence of other, much weaker Fe [i]{} and Fe [ii]{} lines, but the fits are satisfactory overall. The same problem occasionally affects the fits for the Si [ii]{} line (Fig. \[fig:spectraB\]), due to an unidentified weak line that is visible in the later exposures around 1348.75Å. Its profile 