I have a button element that I've created like so:

$submit = new Zend_Form_Element_Button('submit');
$submit->setLabel('My Button');
$submit->setDecorators(array(
    'ViewHelper',
     array('HtmlTag', array('tag' => 'li'))
));
$submit->setAttrib('type', 'submit');

This generates the following HTML:

<li>
    <label for="submit" class="optional">My Button</label> 
    <button name="submit" id="submit" type="submit">My Button</button>
</li>

I would like to wrap the inside of the button with a <span>, like this:

<button...><span>My Button</span></button>

What is the best way to do this using Zend_Form?