code: def get_allowed(allow, disallow):
      
      if allow is None and disallow is None:
          return SUMO_VEHICLE_CLASSES
      elif disallow is None:
          return allow.split()
      else:
          disallow = disallow.split()
          return tuple([c for c in SUMO_VEHICLE_CLASSES if c not in disallow])
docs: Normalize the given string attributes as a list of all allowed vClasses.