docs: def get_layer(self, class_: Type[L], became: bool=True) -> L:
          """
          Return the first layer of a given class. If that layer is not present,
          then raise a KeyError.
  
          :param class_: class of the expected layer
          :param became: Allow transformed layers in results
          """
  
          try:
              return self._index[class_][0]
          except KeyError:
              if became:
                  return self._transformed[class_][0]
              else:
                  raise