A <code class="computeroutput"><span class="identifier">type</span></code> member that
                defines the type of the key
              </li>
<li class="listitem">
                A member function that returns the key derived a <code class="computeroutput"><span class="identifier">value_type</span></code>,
                either by value or by const-reference.
              </li>
</ul></div>
        </li>
</ul></div>
<p>
      Let's see an example of how a set can be configured as a map indexed by