code: def replace_666(meta_df, convert_neg_666):
      
      if convert_neg_666:
          out_df = meta_df.replace([-666, "-666", -666.0], np.nan)
      else:
          out_df = meta_df.replace([-666, -666.0], "-666")
      return out_df
docs: Replace -666, -666.0, and optionally "-666".
      Args:
          meta_df (pandas df):
          convert_neg_666 (bool):
      Returns:
          out_df (pandas df): updated meta_df