How do I reinstall Unity?

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http://askubuntu.com – I tested out Unity 5, and of course, it broke things, so I tried downgrading by sudo apt-get install ppa-purge sudo ppa-purge ppa:unity-team/staging It removed Unity completely. apt-get install unity seemed to work, but the login screen now offers only Gnome. How do I get back to Unity 4? (HowTos)