lass="code">n</code>th cell of <code class="code">ar</code> is
   empty, <code class="code"><span class="constructor">Some</span> x</code> (where <code class="code">x</code> is a (shallow) copy of the value) if
   it is full.
   In addition 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