Comment 54 for bug 1717040

I tried to follow the procedure presented on https://wiki.ubuntu.com/Testing/EnableProposed,
but got blocked at this stage :

```
sudo apt-get install zstd/xenial-proposed
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Release 'xenial-proposed' for 'zstd' was not found
```

and indeed, a further call to `apt list --upgradable`
does not show `zstd/xenial-proposed` in the list of packages.