protected function addCheckbox($tag)
    {
        if (! isset($this->checkboxes[$tag->getName()])) {
            $this->widgets[]                   = $tag;
            $this->checkboxes[$tag->getName()] = count($this->widgets) - 1;
        } else {
            $index = $this->checkboxes[$tag->getName()];
            if (! is_a($this->widgets[$index], 'SimpleCheckboxGroup')) {
                $previous              = $this->widgets[$index];
                $this->widgets[$index] = new SimpleCheckboxGroup();
                $this->widgets[$index]->addWidget($previous);
            }
            $this->widgets[$index]->addWidget($tag);
        }
    }