protected function _add_condition($type, $fragment, $values = array())
{
$conditions_class_property_name = "_{$type}_conditions";
if (!is_array($values)) {
$values = array($values);
}
array_push($this->{$conditions_class_property_name}, array(self::CONDITION_FRAGMENT => $fragment, self::CONDITION_VALUES => $values));
return $this;
}