3400 Å–9600 Å region of the stack of eighteen redshift-corrected SDSS spectra of the IMP sample, binned in 1 Å bins and truncated in flux density f to enhance the visibility of weak spectral features (bottom row). The coloured boxes indicate the regions shown in the blow-ups in the top row, centred at [FeV] 4227, He II 4686, and C IV 5808 (blue, green, and red insets, respectively). Noteworthy emission lines (both present and absent) are marked by coloured vertical lines. The SDSS wavelengths were converted to vacuum wavelengths following Morton (1991).