span><span class="p">(</span><span class="n">m</span><span class="p">,</span><span class="n">i</span><span class="p">,</span><span class="n">j</span><span class="p">)))&amp;</span>
      <span class="o">/</span> <span class="p">(</span><span class="n">dx</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span class="n">j</span><span class="p">)</span><span class="o">*</span><span class="n">dy</span><span class="p">(</span><span class="n">i</span><span class="p">,</span><span clas