Attribute-sets
    Attribute sets are an association between string keys and a Nix values.
    Keys can only be strings.
    When writing attribute sets you can also use unquoted identifiers as keys.

        nix-repl> s = { foo = "bar"; a-b = "baz"; "123" = "num"; }
        nix-repl> s
            { "123" = "num"; a-b = "baz"; foo = "bar"; }

    For those reading Nix expressions from nixpkgs: do not confuse attribute sets with argument sets used in functions. 

    To access elements in the attribute set:

        nix-repl> s.a-b
            "baz"
        nix-repl> s."123"
            "num"

    Yes, you can use strings to address keys which aren't valid identifiers.

    Inside an attribute set you cannot normally refer to elements of the same attribute set:

        nix-repl> { a = 3; b = a+4; }
            error: undefined variable `a' at (string):1:10

    To do so, use recursive attribute sets:
        https://nixos.org/manual/nix/stable/#idm140737321983744

        nix-repl> rec { a = 3; b = a+4; }
            { a = 3; b = 7; }

    This is very convenient when defining packages, which tend to be recursive attribute sets.