t will be used to build a filter if and only if a 
 value is specified.  Example:
 </p>
 <pre class="prettyprint">
 &lt;requestHandler name="/select" class="solr.SearchHandler"&gt;
   &lt;lst name="invariants"&gt;
     &lt;str name="shipping_fq"&gt;{!frange u=$shipping}shipping_cost&lt;/str&gt;
   &lt;/lst&gt;
   &lt;lst name="defaults"&gt;
     &lt;str name="shipping"&gt;any&lt;/str&gt;
   &lt;/lst&gt;
   &lt;lst name="appends"&gt;
     &lt;str name="fq"&gt;{!switch case='*:*'
                             