span class="special">::=</span> <span class="char">'+'</span> <span class="special">|</span> <span class="char">'!'</span>
<span class="identifier">test_set</span>      <span class="special">::=</span> <span class="identifier">label</span> <span class="special">|</span> <span class="identifier">path</span>
<span class="identifier">label</span>         <span class="special">::=</span> <span class="char">'@'</span> <span class="identifier">identifier</span>
<span class="identifier">path</span>          <span 