docs: def check_required_columns(df, filename, required_columns):
      """
      Ensure that all required columns are present in the given dataframe,
      otherwise raise an exception.
      """
      available_columns = set(df.columns)
      for column_name in required_columns:
          if column_name not in available_columns:
              raise ValueError("FPKM tracking file %s missing column '%s'" % (
                  filename,
                  column_name))