Using perch:if not-exists="perch_item_last" to add comma or closing period in a list

if-not-exists-sample

<perch:repeater id="authors" label="Author(s)" order="2">
<perch:content id="firstname" type="smarttext" chars="1" label="First Name/Initial" required="true" /><perch:if exists="firstname">. </perch:if><perch:content id="middlename" type="smarttext" chars="1" label="Middle Name/Initial"/><perch:if exists="middlename">. </perch:if> <perch:content id="lastname" type="smarttext" label="Last Name" required="true"/><perch:if not-exists="perch_item_last">, <perch:else />. </perch:if>
</perch:repeater>