The kernel message being processed.
        cell : nbformat.NotebookNode
            The cell which is currently being processed.
        cell_index : int
            The position of the cell within the notebook object.

        Returns
        -------
        output : NotebookNode
            The execution output payload (or None for no output).

        Raises
        ------
        CellExecutionComplete
          Once a message arrives which indicates computation completeness.

        r·