/**
  * Call-back function for sorting transitions by their priority. Lower sort index => lower priority.
  * @param WorkflowTransition $a
  * @param WorkflowTransition $b
  */
 private static function sortTransitions(WorkflowTransition $a, WorkflowTransition $b)
 {
     if ($a->getSortIndex() == $b->getSortIndex()) {
         return 0;
     }
     return $a->getSortIndex() < $b->getSortIndex() ? -1 : 1;
 }