docs: def _preamble_cell(path):
      """
      This cell is inserted at the start of each notebook to set the working
      directory to the correct folder.
      """
      code = dedent(
          '''
      # HIDDEN
      # Clear previously defined variables
      %reset -f
      '''.format(path)
      )
      return nbformat.v4.new_code_cell(source=code)