 the same as Loc::value_type</span>
  <span class="k">template</span> <span class="o">&lt;</span><span class="kt">size_t</span> <span class="n">D</span><span class="o">&gt;</span> <span class="k">struct</span> <span class="n">axis</span> <span class="p">{</span>
      <span class="k">typename</span> <span class="n">coord_t</span> <span class="o">=</span> <span class="n">point_t</span><span class="o">::</span><span class="n">axis</span><span class="o">&lt;</span><span class="n">D</span><span class="o">&gt;::