ass="programlisting"><span class="identifier">filter_set</span>    <span class="special">::=</span> <span class="special">(</span><span class="identifier">filter</span> <span class="char">':'</span><span class="special">)*</span> <span class="identifier">filter</span>
<span class="identifier">filter</span>        <span class="special">::=</span> <span class="identifier">relative_spec</span><span class="special">?</span> <span class="identifier">test_set</span>
<span class="identifier">relative_spec</span> <