private function appendAggregatorForm(FormBuilderInterface $builder, $nbAggregators)
    {
        $builder->add('order', 'choice', array(
           'choices' => array_combine(
                 range(1, $nbAggregators),
                 range(1, $nbAggregators)
                 ),
           'multiple' => false,
           'expanded' => false
        ));
        
        $builder->add('position', 'choice', array(
           'choices' => array(
              'row' => 'r',
              'column' => 'c'
           ),
           'choices_as_values' => true,
           'multiple' => false,
           'expanded' => false
        ));
    }