      <em class="parameter"><code>a</code></em> to weight between them. The return
            value is computed as $x \times (1 - a) + y \times a$.
        </p>
        <p>
            The variants of <code class="function">mix</code> where
            <em class="parameter"><code>a</code></em> is <code class="constant">genBType</code> select
            which vector each returned component comes from. For a component
            of <em class="parameter"><code>a</code></em> that is false, the corresponding
