@staticmethod
def _node_key(layer, node_index):
    """Converts a layer and its index to a unique (immutable type) name.

        This function is used internally with `self._network_nodes`.

        # Arguments
            layer: The layer.
            node_index: The layer's position (e.g. via enumerate) in a list of
                nodes.

        # Returns
            The unique name.
        """
    return layer.name + '_ib-' + str(node_index)
