public function setPlaceholder ($placeholder = null)
    {
        if ($placeholder === false)
            $this->usePlaceholder = false;
        else {
            $this->setSelect2Options(array(
                self::SELECT2_OPTION_PLACEHOLDER => $placeholder === null ? $this->label : $placeholder
            ));
            $this->usePlaceholder = true;
        }
        return $this;
    }