      are a particular case of a Boost.Bimap container, where you can view only
          one side of the relationship and can control the constraints of only one
          of the collections. Boost.Bimap allows the user to view the relationship
          from three viewpoints. You can view it from one side, obtaining a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code> compatible container, or you can
          work dire