of <code class="function">mix</code> where <em class="parameter"><code>a</code></em> is <code class="constant">genBType</code>,
            elements for which <em class="parameter"><code>a</code></em>[<span class="emphasis"><em>i</em></span>] is <code class="constant">false</code>, the result for that
            element is taken from <em class="parameter"><code>x</code></em>, and where <em class="parameter"><code>a</code></em>[<span class="emphasis"><em>i</em></span>] is
            <code class="constant">