清理Ubuntu软件包

我的小服务器上一直装了个Ubuntu Desktop，不过安装之后一直没用过GUI，而且各种依赖的包有时也挺烦人的，比如gvfs和tracker自带的systemd user service，我还得手动给若干用户禁用掉。

本来我是想着留个Desktop，万一紧急情况可以上网查查命令。不过有网的话，最差情况我应该也能临时装一个X和浏览器，估计问题不太大。

于是我决定把Ubuntu Desktop换成Ubuntu Server，主要还是把gnome的包都删了。

一番折腾以后，安装包的数量从大约1800降到了800以下。舒服！

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Determine Perspective Lines With Off-page Vanishing Point

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