docs: def drop_constant_column_levels(df):
      """
      drop the levels of a multi-level column dataframe which are constant
      operates in place
      """
      columns = df.columns
      constant_levels = [i for i, level in enumerate(columns.levels) if len(level) <= 1]
      constant_levels.reverse()
  
      for i in constant_levels:
          columns = columns.droplevel(i)
      df.columns = columns