"[Solved] Aggregation and generate Attribute"

Hi !
Maybe I missed something obvious:
It seems to me that it is not possible to postprocess an
attribute coming from an aggregation task.
In the attached workflow I would like to do some calculations
on "sum(attr1)" but it fails. Do I need macros ?
Thanks for some hints in advance !

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.2.008">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="5.2.008" expanded="true" name="Process">
    <process expanded="true" height="431" width="882">
      <operator activated="true" class="generate_data" compatibility="5.2.008" expanded="true" height="60" name="Generate Data" width="90" x="45" y="210">
        <parameter key="target_function" value="random classification"/>
      </operator>
      <operator activated="true" class="aggregate" compatibility="5.2.008" expanded="true" height="76" name="Aggregate" width="90" x="179" y="75">
        <list key="aggregation_attributes">
          <parameter key="att1" value="sum"/>
        </list>
        <parameter key="group_by_attributes" value="label|"/>
      </operator>
      <operator activated="true" class="generate_attributes" compatibility="5.2.008" expanded="true" height="76" name="Generate Attributes" width="90" x="447" y="75">
        <list key="function_descriptions">
          <parameter key="new_attrib" value="sum(att1) * 100"/>
        </list>
      </operator>
      <connect from_op="Generate Data" from_port="output" to_op="Aggregate" to_port="example set input"/>
      <connect from_op="Aggregate" from_port="example set output" to_op="Generate Attributes" to_port="example set input"/>
      <connect from_op="Generate Attributes" from_port="example set output" to_port="result 1"/>
      <connect from_op="Generate Attributes" from_port="original" to_port="result 2"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
      <portSpacing port="sink_result 3" spacing="0"/>
    </process>
  </operator>
</process>