 class="varname">gl_ClipDistance</code>
            that have been enabled via the OpenGL API, or results are undefined. Values written into
            <code class="varname">gl_ClipDistance</code> planes that are not enabled have no effect.
        </p>
        <p>
            In the vertex, tessellation evaluation and geometry languages, a single
            global instance of the <code class="varname">gl_PerVertex</code> named block is available and
            its <code class="varname">gl_ClipDistance</