(the X Athena Widgets) from the `xorg` set.

Summarizing the fragment

```nix
...
inherit x y z;
inherit (src-set) a b c;
...
```

is equivalent to

```nix
...
x = x; y = y; z = z;
a = src-set.a; b = src-set.b; c = src-set.c;
...
```

when used while defining local variables in a let-expression or while
defining a set.

In a `let` expression, `inherit` can be used to selectively bring specific attributes of a set into scope. For example


```nix
let
  x = { a = 1; b = 2; };
  inherit (builtins) attrNames;
in
{
  names = attrNames x;
}
```

is equivalent to

```nix
let
  x = { a = 1; b = 2; };
in
{
  names = builtins.attrNames x;
}
```

both evaluate to `{ names = [ "a" "b" ]; }`.

## Functions

Functions have the following form:

```nix
pattern: body
```

The pattern specifies what the argument of the function must look like,
and binds variables in the body to (parts of) the argument. There are
three kinds of patterns:

  - If a pattern is a single identifier, then the function matches any
    argument. Example:

    ```nix
    let negate = x: !x;
        concat = x: y: x + y;
    in if negate true then concat "foo" "bar" else ""
    ```

    Note that `concat` is a function that takes one argument and returns
    a function that takes another argument. This allows partial
    parameterisation (i.e., only filling some of the arguments of a
    function); e.g.,

    ```nix
    map (concat "foo") [ "bar" "bla" "abc" ]
    ```

    evaluates to `[ "foobar" "foobla" "fooabc" ]`.

  - A *set pattern* of the form `{ name1, name2, …, nameN }` matches a
    set containing the listed attributes, and binds the values of those
    attributes to variables in the function body. For example, the
    function

    ```nix
    { x, y, z }: z + y + x
    ```

    can only be called with a set containing exactly the attributes `x`,
    `y` and `z`. No other attributes are allowed. If you want to allow
    additional arguments, you can use an ellipsis (`...`):

    ```nix
    { x, y, z, ... }: z + y + x
    ```

    This works on any set that contains at least the three named
    attributes.

  - It is possible to provide *default values* for attributes, in
    which case they are allowed to be missing. A default value is
    specified by writing `name ?  e`, where *e* is an arbitrary
    expression. For example,

    ```nix
    { x, y ? "foo", z ? "bar" }: z + y + x
    ```

    specifies a function that only requires an attribute named `x`, but
    optionally accepts `y` and `z`.

  - An `@`-pattern provides a means of referring to the whole value
    being matched:

    ```nix
    args@{ x, y, z, ... }: z + y + x + args.a
    ```

    but can also be written as:

    ```nix
    { x, y, z, ... } @ args: z + y + x + args.a
    ```

    Here `args` is bound to the argument *as passed*, which is further
    matched against the pattern `{ x, y, z, ... }`.
    The `@`-pattern makes mainly sense with an ellipsis(`...`) as
    you can access attribute names as `a`, using `args.a`, which was
    given as an additional attribute to the function.

    > **Warning**
    >
    > `args@` binds the name `args` to the attribute set that is passed to the function.
    > In particular, `args` does *not* include any default values specified with `?` in the function's set pattern.
    >
    > For instance
    >
    > ```nix
    > let
    >   f = args@{ a ? 23, ... }: [ a args ];
    > in
    >   f {}
    > ```
    >
    > is equivalent to
    >
    > ```nix
    > let
    >   f = args @ { ... }: [ (args.a or 23) args ];
    > in
    >   f {}
    > ```
    >
    > and both expressions will evaluate to:
    >
    > ```nix
    > [ 23 {} ]
    > ```

  - All bindings introduced by the function are in scope in the entire function expression; not just in the body.
    It can therefore be used in default values.

    > **Example**
    >
    > A parameter (`x`), is used in the default value for another parameter (`y`):
    >
    > ```nix
    > let
    >   f = { x, y ? [x] }: { inherit y; };