<action type="add">
<relation role="posrel" destinationtype="urls">
<field name="pos">{sum(//[EMAIL PROTECTED]'posrel' and @lastitem='true']/[EMAIL PROTECTED]'pos'])+{$pos}}</field>
</relation>
</action>
It's better to add the related object type in case more than one type of builder is related with posrel. I've updated the documentation some months ago.
http://www.mmbase.org/mmdocs/frontenddevelopers/editwizard/editwizard-reference.html
<action type="create">
<relation role="posrel" type="news">
<field name="pos">{sum(//[EMAIL PROTECTED]'posrel' and @lastitem='true' and object/@type='news']/[EMAIL PROTECTED]'pos'])+1}</field>
</relation>
</action>
Jaco
