docs: def _get_col_items(mapping):
      """Get all column items to format, including both keys of `mapping`
      and MultiIndex levels if any.
      """
      from .variable import IndexVariable
  
      col_items = []
      for k, v in mapping.items():
          col_items.append(k)
          var = getattr(v, 'variable', v)
          if isinstance(var, IndexVariable):
              level_names = var.to_index_variable().level_names
              if level_names is not None:
                  col_items += list(level_names)
      return col_items