"identifier">expression</span></code>
      is, in fact, an object that has a member function <code class="computeroutput"><span class="identifier">parse</span></code>
      that does the work given a source code written in the grammar that we have
      just declared. Yes, it's a calculator. We shall simplify for now by skipping
      the type declarations and the definition of the rule <code class="computeroutput"><span class="identifier">integer</span></code>
      invoked by <code class="computeroutput"