autoclean\n      Like  clean,  autoclean  clears  out the local repository of re-\n      trieved package files. The difference is that  it  only  removes\n      package  files that can no longer be downloaded, and are largely\n      useless. This allows a cache to be maintained over a long period\n      without  it  growing  out  of  control. The configuration option\n      APT::Clean-Installed will prevent installed packages from  being\n      erased if it is set to off.\n