public function quote ($string)
		{
			if (is_null($string)) {
				return 'NULL';

			} else if (is_int($string) || is_float($string)) {
				return $string;

			} else if ($string instanceof Raw || $string instanceof DatabaseQuery) {
				return (string) $string;

			} else {
				return $this->database->quote($string);
			}
		}