public function sort_by( $attribute, $sort_flag = SORT_REGULAR )
	{
		$values = array();

		for ( $i = count( $this ) -1; $i >= 0; --$i ) {
			$values[] = $this[$i]->$attribute;
		}

		$values = array_unique($values);

		sort( $values, $sort_flag );

		$new = new self();

		foreach ( $values as $value ) {
			for ( $i = count( $this ) -1; $i >= 0; --$i ) {
				if ( $value == $this[$i]->$attribute ) {
					$new[] = $this[$i];
				}
			}
		}

		return $new;
	}