">len</code> do
   not designate a valid subarray of <code class="code">ar1</code>, or if <code class="code">off2</code> and <code class="code">len</code>
   do not designate a valid subarray of <code class="code">ar2</code>.<br>
</div>
<br>
<h6 id="6_Weakhashtables">Weak hash tables</h6><br>
<br>
A weak hash table is a hashed set of values.  Each value may
    magically disappear from the set when it is not used by the
    rest of the program any more.  This is normally used to share
    data structures wi