span class="k">return</span> <span class="kc">yes</span> <span class="k">if</span> <span class="o">not</span> <span class="nx">seenSingle</span> <span class="o">and</span> <span class="nx">token</span><span class="p">.</span><span class="nx">fromThen</span>
      <span class="nv">seenSingle  = </span><span class="kc">yes</span> <span class="k">if</span> <span class="nx">tag</span> <span class="k">in</span> <span class="p">[</span><span class="s1">&#39;IF&#39;</span><span class="p">,</span> <span class="s1">