do not define classes or enumerations</li>
<li><a title="cpp/language/namespace" style="text-decoration: none; color: rgb(11, 0, 128); background-image: none; background-position: initial initial; background-repeat: initial initial;">using declarations</a></li>
<li><a title="cpp/language/namespace" style="text-decoration: none; color: rgb(11, 0, 128); background-image: none; background-position: initial initial; background-repeat: initial initial;">using directives</a></li>
<li>exactly one <a title="cpp/la