   A tagged type is a type that has been labelled using a tag. A tag is any
        valid C++ type. In a bimap, the types are always tagged. If you do not specify
        your own tag, the container uses <code class="computeroutput"><span class="identifier">member_at</span><span class="special">::</span><span class="identifier">left</span></code>
        and <code class="computeroutput"><span class="identifier">member_at</span><span class="special">::</span><span class="identifier">right</span></code> to ta