{
  my @cache = (0, 1); # Localized so only &Fib sees it.

  sub Fib($)
  {
    my $index = shift;
    return $cache[$index] if defined $cache[$index];
    $index -= $#cache;
    push @cache, $cache[-1] + $cache[-2] while $index--;
    return $cache[-1];
  }
}