sub Fib($)
{
  my $index = shift;
  my $acc = 0;
  my $tmp = 1;

  while( $index-- )
  {
    # Swap the accumulator and the temp.
    # Use an Xor swap for fun and to amaze your friends.
    $acc^=$tmp^=$acc^=$tmp;

    # Add the two together and store it in the acc.
    $acc += $tmp;
  }

  return $acc;
}