code: def add_value(self, value, row, col):
          
          self.__values[(row, col)] = value
docs: Adds a single value (cell) to a worksheet at (row, col).
          Return the (row, col) where the value has been put.
  
          :param value: Value to write to the sheet.
          :param row: Row where the value should be written.
          :param col: Column where the value should be written.