public function start($name)
    {
        if (\in_array($name, $this->openSlots)) {
            throw new \InvalidArgumentException(sprintf('A slot named "%s" is already started.', $name));
        }

        $this->openSlots[] = $name;
        $this->slots[$name] = '';

        ob_start();
        ob_implicit_flush(0);
    }