Zend Framework - Zend_Form Decorator Issue

class Core_Form extends Zend_Form
{
    protected static $_elementDecorators = array(
        'ViewHelper',
        'Errors',
        array('Label'),
        array('HtmlTag', array('tag' => 'li')),
    );

    public function loadDefaultDecorators()
    {
        $this->setElementDecorators(self::$_elementDecorators);
    }
}

class ExampleForm extends Core_Form
{
    public function init()
    {
        // Example Field
        $example = new Zend_Form_Element_Hidden('example');
        $this->addElement($example);
    }
}

<?php echo $this->exampleForm->example; ?>

<li><input type="hidden" name="example" value=""></li>

class ExampleForm extends Core_Form
    {
    	public function init()
    	{
    		//Example Field
    		$example = new Zend_Form_Element_Hidden('example');
    		$this->addElement($example);
    	}

    	public function loadDefaultDecorators()
        {
        	$this->example->setDecorators(array('ViewHelper'));
        }
    }

<?php echo $this->exampleForm->example->setDecorators(array('ViewHelper')) ; ?>

class M_Form_Element_Hidden extends Zend_Form_Element_Hidden {
   public function init() {
      $this->setDisableLoadDefaultDecorators(true);
      $this->addDecorator('ViewHelper');
      $this->removeDecorator('DtDdWrapper');
      $this->removeDecorator('HtmlTag');
      $this->removeDecorator('Label');
      return parent::init();
   }
}

$element = new M_Form_Element_Hidden('myElement');
$this->addElement($element);

$this->addElement(
  'text',
  'a1',
  array('required' => true, 'validators' => array('Alpha'))
);

$this->setElementDecorators(array('ViewHelper'));

$nombre1 = new Zend_Form_Element_Text(
          'n1',
          array('id'=> 'Nombre1', 'validators' => array('Alpha') )
            );
//$nombre1->setDecorators(array('ViewHelper'));
$this->addElement($nombre1);

//$nombre1->setDecorators(array('ViewHelper'));