how to remove elements from generated weight list

I have a stored weight generated by operator "weight by information gain". I am planning to just keep several specific items in this weight list and remove others. How can I achieve that goal? I think i may use "filter example" operator, but could not get it work. The following is the process

<?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="373" width="720">
      <operator activated="true" class="retrieve" compatibility="5.1.011" expanded="true" height="60" name="Retrieve" width="90" x="62" y="130">
        <parameter key="repository_entry" value="svm_weight"/>
      </operator>
      <operator activated="true" class="weights_to_data" compatibility="5.1.011" expanded="true" height="60" name="Weights to Data" width="90" x="174" y="38"/>
      <operator activated="true" class="filter_examples" compatibility="5.1.011" expanded="true" height="76" name="Filter Examples" width="90" x="386" y="216">
        <parameter key="condition_class" value="missing_attributes"/>
        <parameter key="invert_filter" value="true"/>
      </operator>
      <connect from_op="Retrieve" from_port="output" to_op="Weights to Data" to_port="attribute weights"/>
      <connect from_op="Weights to Data" from_port="example set" to_op="Filter Examples" to_port="example set input"/>
      <portSpacing port="source_input 1" spacing="0"/>
      <portSpacing port="sink_result 1" spacing="0"/>
    </process>
  </operator>
</process>