Also, for convenience, nested objects/attrsets can be shortened. For instance:

    {
        a = {
            b = {
                c = 3;
            };
        };
    }

can be shortened to:

    {
        a.b.c = 3;
    }

This is used quite often in NixOS system configuration files. There you'll find lines like:

    services.openssh.enable = true;

For a list of all system configuration options that NixOS supports by default see (NixOS options)[https://nixos.org/nixos/options.html].