an class="n">where</span> <span class="n">Point2DConcept</span><span class="o">&lt;</span><span class="n">point_t</span><span class="o">&gt;</span><span class="p">;</span>

  <span class="k">typename</span> <span class="n">x_iterator</span> <span class="o">=</span> <span class="n">axis</span><span class="o">&lt;</span><span class="mi">0</span><span class="o">&gt;::</span><span class="n">iterator</span><span class="p">;</span>
  <span class="k">typename</span> <span class="n">y_iterator</span> <span class="o