Nested repeat and xf:delete question ...

Hello list,

I've made this forms. It use nested repeat and works quite well.
My only problem is that I want to put delete trigger  in  nested-repeat
to delete a specific line
but I don't know which value I should put in @at  attribute .... Look at
button  "Delete this feature"

I hope that someone have got the answer or any tips
Regards


            <form:repeat nodeset="concentre:release" id="repeat-release">
                <form:group>
                    <form:label>Release</form:label>
                   
              <form:input ref="concentre:label" >
                       <form:label>Label:</form:label>
                   </form:input>
                   
                    <form:group ref="concentre:features"
appearance="minimal">
                    <form:repeat nodeset="concentre:feature"
id="repeat-features">
                        <form:group>
                            <form:label>Feature</form:label>

                      <form:input ref="concentre:label" >
                        <form:label>Name:</form:label>
                      </form:input>
                      <form:select1 ref="concentre:status"
appearance="minimal">
                        <form:label>Status:</form:label>
                        <form:item><form:label>Todo</form:label> 
<form:value>todo</form:value>  </form:item>
                        <form:item><form:label>Done</form:label>
<form:value>done</form:value> </form:item>
                        <form:item><form:label>Working on</form:label>
<form:value>workingon</form:value> </form:item>
                        <form:item><form:label>Discarded</form:label>
<form:value>discarded</form:value> </form:item>
                      </form:select1>
                     
                      <form:trigger>
                        <form:label>Delete this feature</form:label>
                        <form:delete ev:event="DOMActivate"
nodeset="../concentre:feature" at="1" />
                      </form:trigger>
                  </form:group>
                     </form:repeat>
                    
                    <form:trigger>
                <form:label>Add a feature</form:label>
                <form:insert ev:event="DOMActivate"
nodeset="concentre:feature" at="last()" position="after"/>
                  </form:trigger>

              </form:group>
            </form:group>
            </form:repeat>
                    <form:trigger>
                <form:label>Add release</form:label>
                <form:insert ev:event="DOMActivate"
nodeset="concentre:release" at="last()" position="after"/>
                  </form:trigger>