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function hook_form_alter(&$form, &$form_state, $form_id) {

  $substitutions = array(
    array(
      'placeholder' => '<!--form-item-action--submit-->',
      'field_name' => 'actions',
      'row_id' => 'submit',
    ),
    array(
      'placeholder' => '<!--form-item-action--checkout-->',
      'field_name' => 'actions',
      'row_id' => 'checkout',
    ),
  );
  
  // add placeholders to output 

  $form['#substitutions']['#value'] = array_merge($form['#substitutions']['#value'], $substitutions);
}