docs: def within_range(df, items=None):
      """
      Assert that a DataFrame is within a range.
  
      Parameters
      ==========
      df : DataFame
      items : dict
          mapping of columns (k) to a (low, high) tuple (v)
          that ``df[k]`` is expected to be between.
      """
      for k, (lower, upper) in items.items():
          if (lower > df[k]).any() or (upper < df[k]).any():
              raise AssertionError
      return df