1from time to time run the following commands to remove stuff from old generations
 2```
 3# list old generations
 4sudo nix-env --list-generations --profile /nix/var/nix/profiles/system
 5
 6# remove old generations
 7sudo nix-env --profile /nix/var/nix/profiles/system --delete-generations 9 10 11 12 13 X...
 8
 9
10# to remove old programs
11nix-collect-garbage --delete-old
12```