="special">.</span><span class="identifier">first</span></code> hardcoded as the key part and <code class="computeroutput"><span class="identifier">pair</span><span class="special">.</span><span class="identifier">second</span></code>
      hardcoded as the <code class="computeroutput"><span class="identifier">mapped_type</span></code>,
      which is limiting and also not very useful in practice. Any intrusive associative
      container can be used like a map using <a class="link" href="advanced_lookups_i