option override

form.php

<?php
class BicycleType extends AbstractType 
{
    public function buildForm($formBuilder)
    {
       $formBuilder->add('wheels', 'choice', array('choices' => array(1, 2)));
    }
}

class Tricycle extends BicycleType 
{
    public function buildForm($formBuilder)
    {
       parent::buildForm($formBuilder);

      // how to add choices option?
    }

}