tart with
<code>'ref_'</code> to note the reference semantic, for example, <code><a href="add.html">add()</a></code> and <code><a href="add.html">ref_add()</a></code>.</p>
<ul><li><p><code>deque(...)</code> initializes and returns an object of class <code>Deque</code></p></li>
</ul><ul><li><p><code>as.deque(x)</code> coerces <code>x</code> to a deque.</p></li>
</ul><ul><li><p><code>is.deque(x)</code> returns <code>TRUE</code> if <code>x</code> is of class <code>Deque</code>
and <code>FALSE</code> otherwise.