n</code> characters from
   the current contents of the buffer <code class="code">src</code>, starting at offset <code class="code">srcoff</code>
   to <code class="code">dst</code>, starting at character <code class="code">dstoff</code>.</p>

<p>Raise <code class="code"><span class="constructor">Invalid_argument</span></code> if <code class="code">srcoff</code> and <code class="code">len</code> do not designate a valid
   range of <code class="code">src</code>, or if <code class="code">dstoff</code> and <c