gistfile1.txt

<tool id="template" name="template" version="0.0.1">
    <command/>
    <inputs>
        <conditional name="conditional">
            <param name="test_parameter" type="select">
                <option value="yes">Yes</option>
                <option value="no">No</option>
            </param>
            <when value="yes">
                <param name="a" type="data"/>
            </when>
            <when value="no">
                <param name="b" type="data"/>
            </when>
        </conditional>
    </inputs>
    <outputs/>
</tool>