mply generates the comma <code class="computeroutput"><span class="char">','</span></code>. In this case, the Kleene Star is modifying
          a more complex generator, namely, the one generated by the expression:
        </p>
<pre class="programlisting"><span class="special">(</span><span class="identifier">lit</span><span class="special">(</span><span class="char">','</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">double_</span><span class="special">)