  <ul>
          <li><a href="#x_mem_fun">Variants for other types of member functions</a></li>
        </ul>
      </li>
      <li><a href="#global_fun"><code>global_fun</code></a></li>
    </ul>
  </li>
  <li><a href="#user_defined_key_extractors">User-defined key extractors</a></li>
  <li><a href="#composite_keys">Composite keys</a>
    <ul>
      <li><a href="#composite_keys_hash">Composite keys and hashed indices</a></li>
    </ul>
  </li>
  <li><a href="#key">C++17 terse key specification syntax</a></