function common_items(&$vector)
{
// for compatibility, we don't use pass by reference
//$keys = array_intersect_key(&$this->values, &$vector->values);
$keys = array_intersect_key($this->values, $vector->values);
if (count($keys) == 0) {
return NULL;
}
$array_a = array();
$array_b = array();
foreach ($keys as $key => $value) {
$array_a[] = $this->values[$key];
$array_b[] = $vector->values[$key];
}
$subset = array();
$subset[] = Vector::wrap('RealVector', $array_a);
$subset[] = Vector::wrap('RealVector', $array_b);
return $subset;
}