 alternative to build a useful map-like lookup interfaces
      without forcing users to define <code class="computeroutput"><span class="identifier">value_type</span></code>s
      containing <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span></code>-like classes. The option is called
      <code class="computeroutput"><a class="link" href="../boost/intrusive/key_of_value.html" title="Struct template key_of_value">boost::intrus