How to run a specific phpunit xml testsuite?

how can i choose a specific testsuite to be executed?

$ phpunit --configuration config.xml

config.xml:

<testsuites>
    <testsuite name="Library">
        <directory>library</directory>
    </testsuite>
    <testsuite name="XXX_Form">
        <file>library/XXX/FormTest.php</file>
        <directory>library/XXX/Form</directory>
    </testsuite>
</testsuites>