public function setIndex($index)
	{
		if ( ! is_numeric($index) || ! is_int(+$index) || $index < 0) {
			throw new \InvalidArgumentException(
				__METHOD__."() expects parameter one, index, to be a positive "
				 ."integer or zero"
			);
		}
		
		$this->index = $index;
		
		return $this;
	}