protected function addConnectionOptionsNode()
    {
        $builder = new TreeBuilder();
        $node = $builder->root('options');

        $node
            ->performNoDeepMerging()
            ->addDefaultsIfNotSet() // adds an empty array of omitted
            // options go into the Mongo constructor
            // http://www.php.net/manual/en/mongo.construct.php
            ->children()
                ->booleanNode('connect')->end()
                ->scalarNode('persist')->end()
                ->scalarNode('timeout')->end()
                ->booleanNode('replicaSet')->end()
                ->scalarNode('username')->end()
                ->scalarNode('password')->end()
            ->end()
        ->end();

        return $node;
    }