1520 [$\rm{\AA} \;$]{}minimum of the trough in the HiBAL composite difference spectrum). The structure in the composite spectra troughs is particularly interesting since, as described above, the construction of composites tends to average out any structure that is not persistent from quasar to quasar.

There is absorption just blueward of the +\] emission-line peak in both the HiBAL and LoBAL composite spectra. In the LoBAL composite difference spectrum, the absorption is double-troughed (with minima near $1356\,{\rm \AA}$ and $1386\,{\rm
\AA}$ and separated by a peak near $1370\,{\rm \AA}$). We see no obvious trend in the separation of the double troughs in , and that are seen in the LoBAL composite difference spectrum, but for all three of these systems the peak between the troughs is roughly $6000\,{\rm km\,s^{-1}}$ from the expected emission-line center, which is suggestive of the radiative acceleration “ghost of Ly$\alpha$” effect described by (1996).

Toward the bluer end of the spectra, the continuum fits are not nearly as good and it becomes difficult to tell whether the features that we see are the result of excess emission or absorption. Nevertheless, there are a few notable features. The LoBAL composite difference spectrum displays absorption just blueward of each 