span><span class="p">)</span>
  <span class="c">!! Returns a centered-difference of a 1-d array with periodic boundary</span>
  <span class="c">!! conditions. This procedure is overloaded by the generic procedure `diff`.</span>
  <span class="kt">real</span><span class="p">(</span><span class="nb">kind</span><span class="o">=</span><span class="n">rk</span><span class="p">),</span><span class="k">dimension</span><span class="p">(:),</span><span class="k">intent</span><span class="p">(</span><span class="n">