I recently installed the Cinnamon Desktop environment and after restarting noticed that the Unity and Unity 2D session options had disappeared from LightDM's session selector.
I removed and purged all Cinnamon packages, but that didn't work. So then I reinstalled all unity-related packages to no avail either. I then removed, purged, and then reinstalled unity and unity-2d to no effect. How can this be fixed?