code: def is_hidden_container(self, key, val):
          
  
          if key in ("layers", "classes", "styles", "symbols", "labels",
                     "outputformats", "features", "scaletokens",
                     "composites") and isinstance(val, list):
              return True
          else:
              return False
docs: The key is not one of the Mapfile keywords, and its
          values are a list