ructor">List</span>.rev</code></a><code class="code"> l1 @ l2</code>, but <code class="code">rev_append</code> is
   tail-recursive and more efficient.<br>
</div>
<pre><span id="VALconcat"><span class="keyword">val</span> concat</span> : <code class="type">'a list list -&gt; 'a list</code></pre><div class="info">
Concatenate a list of lists.  The elements of the argument are all
   concatenated together (in the same order) to give the result.
   Not tail-recursive
   (length of the argument + length of the 