code: def _preprocess(df):
      
  
      df = df.stack()
  
      df.index.rename(["id", "time"], inplace=True)  # .reset_index()
      df.name = "value"
      df = df.reset_index()
  
      return df
docs: given a DataFrame where records are stored row-wise, rearrange it
      such that records are stored column-wise.