docs: def isIn(row, col_name, arg, dm, df, con=None):
      """
      row[col_name] must contain a value from another column.
      If not, return error message.
      """
      #grade = df.apply(func, args=(validation_name, arg, dm), axis=1)
      cell_value = row[col_name]
      cell_value = str(cell_value)
      if not cell_value:
          return None
      elif cell_value == 'None':
          return None
      elif cell_value == 'nan':
          return None
      elif not con:
          return None
      # if it's in another table
      cell_values = [v.strip(" ") for v in cell_value.split(":")]
      if "." in arg:
          table_name, table_col_name = arg.split(".")
          if table_name not in con.tables:
              return None
              #return "Must contain a value from {} table. Missing {} table.".format(table_name, table_name)
          if table_col_name not in con.tables[table_name].df.columns:
              return '{} table is missing "{}" column, which is required for validating "{}" column'.format(table_name, table_col_name, col_name)
          possible_values = con.tables[table_name].df[table_col_name].unique()
          for value in cell_values:
              if value not in possible_values:
                  trunc_possible_values = [val.replace(' ', '') for val in possible_values if val]
                  trunc_cell_value = cell_value.replace(' ', '')
                  if trunc_cell_value not in trunc_possible_values:
                      if trunc_cell_value != value:
                          return 'This value (long): "{}" is not found in: {} column in {} table.  Also (short): {} is not in {}'.format(value, table_col_name, table_name, trunc_cell_value, arg)
                      else:
                          return 'This value: "{}" is not found in: {} column in {} table'.format(value, table_col_name, table_name)
                      break
      # if it's in the present table:
      else:
          possible_values = df[arg].unique()
          for value in cell_values:
              if value not in possible_values:
                  return 'This value: "{}" is not found in: {} column'.format(value, arg)
                  break
      return None