Quartet music (Atari ST / Amiga), the Microdeal sample sequencer.
A Quartet song references a .set instrument definition alongside the tune
itself. The .set files are companion files, not songs — they are listed in
data/misc/not_supported_extensions.txt so they never appear as index rows, and
are fetched automatically as secondary files when a tune plays.
UADE also carries Quartet / Quartet-PSG / Quartet-ST eagleplayers; see uadeplugin.