protected function getConnections(array $config)
    {
        if (!isset($config['connections'])) {
            throw new \RuntimeException('Configuration must contain a "connections" key');
        }

        if (!is_array($config['connections']) || !$config['connections']) {
            throw new \RuntimeException('Configuration "connections" key must reference a non-empty array');
        }

        $connections = array_filter(
            $config['connections'],
            function ($connection) {
                return $connection instanceof ConnectionInterface;
            }
        );
        if (count($connections) != count($config['connections'])) {
            throw new \RuntimeException(
                'All configuration "connections" array values must implement \Phergie\Irc\ConnectionInterface'
            );
        }

        return $connections;
    }