span class="p">(</span><span class="n">tr</span><span class="p">,</span><span class="n">ti</span><span class="p">,</span><span class="n">tr</span><span class="p">,</span><span class="n">ti</span><span class="p">)</span>
<span class="w">      </span><span class="n">yr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="o">-</span><span class="p">(</span><span class="n">ti</span><span class="o">/</span><span class="mf">2.0d0</span><span class="p">)</span>
<span class="w"