[SOLVED] how to generate constant attribute that is function of example set

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<process version="5.1.011">
  <context>
    <input/>
    <output/>
    <macros/>
  </context>
  <operator activated="true" class="process" compatibility="5.1.011" expanded="true" name="Process">
    <process expanded="true" height="386" width="748">
      <operator activated="true" class="generate_data" compatibility="5.1.011" expanded="true" height="60" name="Generate Data" width="90" x="45" y="30"/>
      <operator activated="true" class="generate_id" compatibility="5.1.011" expanded="true" height="76" name="Generate ID" width="90" x="45" y="120"/>
      <operator activated="true" class="set_role" compatibility="5.1.011" expanded="true" height="76" name="Set Role" width="90" x="45" y="210">
        <parameter key="name" value="id"/>
        <list key="set_additional_roles"/>
      </operator>
      <operator activated="true" class="select_attributes" compatibility="5.1.011" expanded="true" height="76" name="Select Attributes" width="90" x="45" y="300">
        <parameter key="attribute_filter_type" value="subset"/>
        <parameter key="attributes" value="|id"/>
        <parameter key="include_special_attributes" value="true"/>
      </operator>
      <operator activated="true" class="extract_macro" compatibility="5.1.011" expanded="true" height="60" name="Extract Macro" width="90" x="179" y="30">
        <parameter key="macro" value="mean"/>
        <parameter key="macro_type" value="statistics"/>
        <parameter key="attribute_name" value="id"/>
      </operator>
      <operator activated="true" class="generate_attributes" compatibility="5.1.011" expanded="true" height="76" name="Generate Attributes" width="90" x="179" y="120">
        <list key="function_descriptions">
          <parameter key="difference" value="id-%{mean}"/>
        </list>
      </operator>
      <connect from_op="Generate Data" from_port="output" to_op="Generate ID" to_port="example set input"/>
      <connect from_op="Generate ID" from_port="example set output" to_op="Set Role" to_port="example set input"/>
      <connect from_op="Set Role" from_port="example set output" to_op="Select Attributes" to_port="example set input"/>
      <connect from_op="Select Attributes" from_port="example set output" to_op="Extract Macro" to_port="example set"/>
      <connect from_op="Extract Macro" from_port="example set" to_op="Generate Attributes" to_port="example set input"/>
      <connect from_op="Generate Attributes" from_port="example set output" to_port="result 1"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
      <portSpacing port="sink_result 2" spacing="0"/>
    </process>
  </operator>
</process>