code: def extrema(self, x0, y0, w, h):
          
          minimum = 9223372036854775807
          maximum = 0
          for y in range(y0, y0 + h):
              for x in range(x0, x0 + w):
                  value = self[x, y]
                  if value != self.filler:
                      minimum = min(minimum, value)
                      maximum = max(maximum, value)
          return minimum, maximum
docs: Returns the minimum and maximum values contained in a given area.
  
          :param x0: Starting x index.
          :param y0: Starting y index.
          :param w:  Width of the area to scan.
          :param h:  Height of the area to scan.
          :return:   Tuple containing the minimum and maximum values of the given area.