implication](https://en.wikipedia.org/wiki/Truth_table#Logical_implication)
    Boolean operation.

2.  This is a more subtle condition: if Subversion is built with Apache
    (`httpServer`) support, then the Expat library (an XML library) used
    by Subversion should be same as the one used by Apache. This is
    because in this configuration Subversion code ends up being linked
    with Apache code, and if the Expat libraries do not match, a build-
    or runtime link error or incompatibility might occur.

3.  This assertion says that in order for Subversion to have SSL support
    (so that it can access `https` URLs), an OpenSSL library must be
    passed. Additionally, it says that *if* Apache support is enabled,
    then Apache's OpenSSL should match Subversion's. (Note that if
    Apache support is not enabled, we don't care about Apache's
    OpenSSL.)

4.  The conditional here is not really related to assertions, but is
    worth pointing out: it ensures that if SSL support is disabled, then
    the Subversion derivation is not dependent on OpenSSL, even if a
    non-`null` value was passed. This prevents an unnecessary rebuild of
    Subversion if OpenSSL changes.

## With-expressions

A *with-expression*,

```nix
with e1; e2
```

introduces the set *e1* into the lexical scope of the expression *e2*.
For instance,

```nix
let as = { x = "foo"; y = "bar"; };
in with as; x + y
```

evaluates to `"foobar"` since the `with` adds the `x` and `y` attributes
of `as` to the lexical scope in the expression `x + y`. The most common
use of `with` is in conjunction with the `import` function. E.g.,

```nix
with (import ./definitions.nix); ...
```

makes all attributes defined in the file `definitions.nix` available as
if they were defined locally in a `let`-expression.

The bindings introduced by `with` do not shadow bindings introduced by
other means, e.g.

```nix
let a = 3; in with { a = 1; }; let a = 4; in with { a = 2; }; ...
```

establishes the same scope as

```nix
let a = 1; in let a = 2; in let a = 3; in let a = 4; in ...
```

Variables coming from outer `with` expressions *are* shadowed:

```nix
with { a = "outer"; };
with { a = "inner"; };
a
```

Does evaluate to `"inner"`.

## Comments

- Inline comments start with `#` and run until the end of the line.

  > **Example**
  >
  > ```nix
  > # A number
  > 2 # Equals 1 + 1
  > ```
  >
  > ```console
  > 2
  > ```

- Block comments start with `/*` and run until the next occurrence of `*/`.

  > **Example**
  >
  > ```nix
  > /*
  > Block comments
  > can span multiple lines.
  > */ "hello"
  > ```
  >
  > ```console
  > "hello"
  > ```

  This means that block comments cannot be nested.

  > **Example**
  >
  > ```nix
  > /* /* nope */ */ 1
  > ```
  >
  > ```console
  > error: syntax error, unexpected '*'
  >
  >        at «string»:1:15:
  >
  >             1| /* /* nope */ *
  >              |               ^
  > ```

  Consider escaping nested comments and unescaping them in post-processing.

  > **Example**
  >
  > ```nix
  > /* /* nested *\/ */ 1
  > ```
  >
  > ```console
  > 1
  > ```