any way to retrieve the index for a specific attribute name?

xquery version "3.0";
declare namespace file="http://expath.org/ns/file";
declare namespace index="http://basex.org/modules/index";

let $periods := index:attributes("13F")
return file:write('/var/www/appusec3.jahtoe.com/xml/periods.xml', $periods)


<entry count="57">000032</entry>
<entry count="48">000033</entry>
<entry count="48">000034</entry>
<entry count="38">000035</entry>
<entry count="40">000036</entry>
<entry count="35">000037</entry>

I specify 4 attribute to be in the index

<commands>
    <set option='attrinclude'>accno,filingDate,form13FFileNumber,periodOfReport</set>
    <create-db name='13F'/>   
    <info-db/>
</commands>

I just require the index values for periodOfReport