不能sudo apt-get update：
Ign:8 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release.gpg
Reading package lists... Done
W: GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release: The following signatures were invalid: EXPKEYSIG 5A7D1D38BEB6D886 home:Horst3180 OBS Project <home:Horst3180@build.opensuse.org>
E: The repository 'http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release' is not signed.
从无数次尝试中，我得到
1 signature not checked due to a missing key
我的尝试是：
sudo apt-key adv --keyserver hkp://keys.gnupg.net:80 --recv-keys 5A7D1D38BEB6D886
Executing: /tmp/apt-key-gpghome.f5yVraTkTa/gpg.1.sh --keyserver hkp://keys.gnupg.net:80 --recv-keys 5A7D1D38BEB6D886
gpg: key 5A7D1D38BEB6D886: 1 signature not checked due to a missing key
gpg: key 5A7D1D38BEB6D886: "home:Horst3180 OBS Project <home:Horst3180@build.opensuse.org>" not …