docs: def add(self, src_cls, module, package=None):
          """
          Add layer class to model. This method may be overloaded by layer.
  
          :param src_cls: layer class to add, should not start with underscores
          :type src_cls: str
          :param module: Python module that contains layer class
          :type module: str
          :param package: optional package containing module with layer class
          :type package: str
          :raises: :exc:`~exceptions.NotImplementedError`
          """
          # import module containing the layer class
          mod = importlib.import_module(module, package)
          # get layer class definition from the module
          self.sources[src_cls] = getattr(mod, src_cls)