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.\nAlso check out man apt.conf.