docs: def preview(df,preview_rows = 20):#,preview_max_cols = 0):
      """ Returns a preview of a dataframe, which contains both header
      rows and tail rows.
      """
      if preview_rows < 4:
          preview_rows = 4
      preview_rows = min(preview_rows,df.shape[0])
      outer = math.floor(preview_rows / 4)
      return pd.concat([df.head(outer),
                        df[outer:-outer].sample(preview_rows-2*outer),
                        df.tail(outer)])