Here is a Nix expression for the Subversion package that shows how
assertions can be used:.

```nix
{ localServer ? false
, httpServer ? false
, sslSupport ? false
, pythonBindings ? false
, javaSwigBindings ? false
, javahlBindings ? false
, stdenv, fetchurl
, openssl ? null, httpd ? null, db4 ? null, expat, swig ? null, j2sdk ? null
}:

assert localServer -> db4 != null; ①
assert httpServer -> httpd != null && httpd.expat == expat; ②
assert sslSupport -> openssl != null && (httpServer -> httpd.openssl == openssl); ③
assert pythonBindings -> swig != null && swig.pythonSupport;
assert javaSwigBindings -> swig != null && swig.javaSupport;
assert javahlBindings -> j2sdk != null;

stdenv.mkDerivation {
  name = "subversion-1.1.1";
  ...
  openssl = if sslSupport then openssl else null; ④
  ...
}
```

The points of interest are:

1.  This assertion states that if Subversion is to have support for
    local repositories, then Berkeley DB is needed. So if the Subversion
    function is called with the `localServer` argument set to `true` but
    the `db4` argument set to `null`, then the evaluation fails.

    Note that `->` is the [logical
    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