 to have an <code class="computeroutput"><span class="keyword">operator</span><span class="special">()</span></code>
          with 3 arguments. Since we don't care about the other two, we can use
          <code class="computeroutput"><span class="identifier">unused_type</span></code> for these.
          We'll see more of <code class="computeroutput"><span class="identifier">unused_type</span></code>
          elsewhere. Get used to it. <code class="computeroutput"><span class="identifier">unused_type</sp