span class="emphasis"><em>oldof</em></span></code><code class="computeroutput"><span class="special">(</span><span class="identifier">x</span><span class="special">)</span></code>).
      Note that postconditions shall be checked only when the execution of the function
      body did not throw an exception.
    </p>
<p>
      Now let's program this function and its contract using this library (see <a href="../../example/features/introduction.cpp" target="_top"><code class="literal">introduction.cpp</code></