public function addBuilder(ManagerBuilder $builder)
    {
        $builderName = (string) $builder->getName();
        if ($builderName === '') {
            throw new \RuntimeException('Only named manager builders allowed');
        }

        if (array_key_exists($builderName, $this->builders)) {
            throw new \RuntimeException(sprintf('"%s" manager builder is already registered', $builderName));
        }

        $this->builders[$builderName] = $builder;
    }