...)</code> but adds by reference.</p></li>
</ul><ul><li><p><code>peek(x, default = NULL)</code> peek at last element. If <code>x</code> is empty, return
<code>default</code>.</p></li>
<li><p><code>peekleft(x, default = NULL)</code> peek at first element. If <code>x</code> is empty,
return <code>default</code>.</p></li>
</ul><ul><li><p><code>ref_pop(.x)</code> pop last element. If <code>.x</code> is empty, an error is given.</p></li>
<li><p><code>ref_popleft(.x)</code> pop first element. If <code>.x</code> 