These interiors are UNUSED; meaning no mission contains a .dif that points to them:

colcylinder.dif
pirateship.dif
railtri.dif
sign_arrow_red_left.dif
sign_arrow_red_right.dif
TitiriteroPlacement.dif
tubetri_2.dif

These missions are likely to be deleted or moved to other subdirectory,
so the .dif for them are found in this subdirectory for now:

flatgrass.dif
gmdemo.dif
lapsTestMission.dif
TheDragonRoom.dif
TheDragonRoom3.dif
trimring_upright.dif
