how to hook checkout pane defined in another module? I try to make a small change to Drupal Commerce Coupon pane (add a div to it). I tried to hook this function:

    function commerce_coupon_pane_checkout_form($form, &amp;$form_state, $checkout_pane, $order) {
              // Allow to replace pane content with ajax calls.
              $pane_form = array(
                '#prefix' =&gt; '&lt;div id="commerce-checkout-coupon-ajax-wrapper"&gt;',
                '#suffix' =&gt; '&lt;/div&gt;',
              );
        (..)
}


To do this I created a function in my module: 

function &lt;mymodule&gt;_form_commerce_coupon_pane_alter(&amp;$form, &amp;$form_state, $form_id) {
        $form['#prefix'] = '&lt;div id="collapsable-panel"&gt;&lt;p&gt;You have coupon? Add it here.&lt;/p&gt;' + $form['#prefix'];
        $form['#suffix'] = $form['#suffix'] + '&lt;/div&gt;';
    }


but it do not work. I think that there should be other way to hook checkout pane but I cannot figure out how to do this.
commerce_checkout_form_PAGE is a form ID. Use it inside of HOOK_form_FORM_ID_alter to edit a particular page of the checkout process. Like so:

function MYMODULE_form_commerce_checkout_form_checkout_alter(&amp;$form, &amp;$form_state, $form_id) {
    $form['#prefix'] = '&lt;div id="collapsable-panel"&gt;&lt;p&gt;You have coupon? Add it here.&lt;/p&gt;' + $form['#prefix'];
    $form['#suffix'] = $form['#suffix'] + '&lt;/div&gt;';
}


Alternatively, you can also do a generic alter and check the build info in $form_state:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state) {
  if ($form_state['build_info']['base_form_id'] == 'commerce_checkout_form') {
    // We're in checkout.
    $form[PANE_ID] ... 
  }
}