$ rm /nix/var/nix/profiles/default-9-link
$ nix-env --list-generations
[...]
   8   2014-07-28 10:23:24
  10   2014-08-20 12:47:16   (current)
$ nix-collect-garbage
[...]
$ ls /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17
ls: cannot access /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17: No such file or directory