            component of <em class="parameter"><code>x</code></em> is returned. For a
            component of <em class="parameter"><code>a</code></em> that is true, the
            corresponding component of <em class="parameter"><code>y</code></em> is returned.
            Components of <em class="parameter"><code>x</code></em> and
            <em class="parameter"><code>y</code></em> that are not selected are allowed to be
            invalid floating-point values and will have no effect on the
        