 class="code">add_substring b s ofs len</code> takes <code class="code">len</code> characters from offset
   <code class="code">ofs</code> in string <code class="code">s</code> and appends them at the end of buffer <code class="code">b</code>.<br>
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<pre><span id="VALadd_subbytes"><span class="keyword">val</span> add_subbytes</span> : <code class="type"><a href="Buffer.html#TYPEt">t</a> -&gt; bytes -&gt; int -&gt; int -&gt; unit</code></pre><div class="info ">
<code class="code">add_subbytes b s ofs l