turn lhs += rhs;
}
</pre>
    The difference to the first implementation is that <code>lhs</code> is
    not taken as a constant reference used to create a copy; instead,
    <code>lhs</code> is a by-value parameter, thus it is already the copy
    needed. This allows another optimization (12.2/2) for some cases.
    Consider <code>a&nbsp;+&nbsp;b&nbsp;+&nbsp;c</code> where the result of
    <code>a&nbsp;+&nbsp;b</code> is not copied when used as <code>lhs</code>
    when adding <code>c</code>. This is more