class="special">::</span><span class="identifier">tag</span></code>,
              defining <code class="computeroutput"><span class="identifier">point_tag</span></code>
              as type
            </li>
<li class="listitem">
              there must be a specialization of <code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">coordinate_type</span></code>,
              defining the type of its coordinates
            </li>
<li class