de class="code">v</code> as
   argument at some point between the first time <code class="code">v</code> becomes unreachable
   (including through weak pointers) and the time <code class="code">v</code> is collected by
   the GC. Several functions can
   be registered for the same value, or even several instances of the
   same function.  Each instance will be called once (or never,
   if the program terminates before <code class="code">v</code> becomes unreachable).
<p>

   The GC will call the finalisatio