public function setPosition( $value )
	{
		if( $value == $this->getPosition() ) { return; }

		if( $value !== null && $value < 1 ) {
			throw new MShop_Order_Exception( sprintf( 'Order product position "%1$s" must be greater than 0', $value ) );
		}

		$this->_values['pos'] = ( $value !== null ? (int) $value : null );
		$this->setModified();
	}