re we proceed, let me introduce a helper class, the x3::error_handler.
        It is utility class that provides <a href="https://clang.llvm.org/" target="_top">Clang</a>
        style error reporting which gives you nice reports such as the following:
      </p>
<pre class="programlisting">In line 16:
Error! Expecting: person here:
    'I am not a person!'    &lt;--- this should be a person
____^_
</pre>
<p>
        We'll see later that this error message is exactly what this example emits.
      </p>
<p>
