code: def add_layers(self, *layers):
  		
  		for layer in layers:
  			if layer.name() in self.__layers.keys():
  				raise ValueError('Layer "%s" already exists' % layer.name())
  			self.__layers[layer.name()] = layer
docs: Append given layers to this onion
  
  		:param layers: layer to add
  		:return: None