to pitfalls with mutable values, the interesting
   difference with <code class="code">get</code> is that <code class="code">get_copy</code> does not prevent
   the incremental GC from erasing the value in its current cycle
   (<code class="code">get</code> may delay the erasure to the next GC cycle).
   Raise <code class="code"><span class="constructor">Invalid_argument</span> <span class="string">"Weak.get"</span></code> if <code class="code">n</code> is not in the range
   0 to <a href="Weak.html#VALleng