我想降级 GNOME，因为我的 Ubuntu 16.04 上的 3.20 太乱了。互联网上有无数的资源（包括问库本图）说明如何做到这一点，而它总是：
sudo ppa-purge ppa:gnome3-team/gnome3-staging
sudo ppa-purge ppa:gnome3-team/gnome3
（也可选择sudo ppa-purge ppa:gnome3-team/gnome3-next）
但是，我对此毫无进展。
$ sudo ppa-purge ppa:gnome3-team/gnome3-staging：
Updating packages lists
W: GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A7D1D38BEB6D886
W: The repository 'http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release' is not signed.
W: The repository 'http://ppa.launchpad.net/cardapio-team/cardapio-ppa/ubuntu xenial Release' does not have a Release file.
W: The repository 'http://ppa.launchpad.net/cardapio-team/unstable/ubuntu xenial Release' does not have a Release file.
W: The repository 'http://ppa.launchpad.net/synapse-core/ppa/ubuntu xenial Release' does not have a Release file.
E: Failed to fetch http://ppa.launchpad.net/cardapio-team/cardapio-ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://ppa.launchpad.net/cardapio-team/unstable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://ppa.launchpad.net/synapse-core/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
Warning: apt-get update failed for some reason
$ sudo ppa-purge ppa:gnome3-team/gnome3：
Updating packages lists
W: The repository 'http://ppa.launchpad.net/cardapio-team/cardapio-ppa/ubuntu xenial Release' does not have a Release file.
W: The repository 'http://ppa.launchpad.net/cardapio-team/unstable/ubuntu xenial Release' does not have a Release file.
W: The repository 'http://ppa.launchpad.net/synapse-core/ppa/ubuntu xenial Release' does not have a Release file.
W: GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A7D1D38BEB6D886
W: The repository 'http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04 Release' is not signed.
E: Failed to fetch http://ppa.launchpad.net/cardapio-team/cardapio-ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://ppa.launchpad.net/cardapio-team/unstable/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Failed to fetch http://ppa.launchpad.net/synapse-core/ppa/ubuntu/dists/xenial/main/binary-amd64/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
Warning: apt-get update failed for some reason
GNOME 在重启后保持相同的版本。抱歉，如果这只是谷歌搜索部分输出的问题，我还尝试搜索有关ppa-purge失败的一般信息。我是一个低于平均水平的最终用户。我很乐意提供更多信息。
按照要求，$ sudo apt-get update输出，并$ sudo apt-get upgrade输出：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.