code: def add_code_cell(cells, source):
      
      from nbformat.v4.nbbase import new_code_cell
      n_code_cells = len([c for c in cells if c['cell_type'] == 'code'])
      cells.append(new_code_cell(source=source, execution_count=n_code_cells+1))
docs: Add a code cell containing `source` to the notebook.