public function createPlaceholder($value)
    {
        if (($key = array_search($value, $this->all()))) {
            $placeholder = $key;
        } else {
            $placeholder = $this->createUniquePlaceholder();
        }

        $value = $this->removeNestedPlaceholders($value);
        $this->items[$placeholder] = $value;

        return $placeholder;
    }