code: def __step6(self):
          
          minval = self.__find_smallest()
          for i in range(self.n):
              for j in range(self.n):
                  if self.row_covered[i]:
                      self.C[i][j] += minval
                  if not self.col_covered[j]:
                      self.C[i][j] -= minval
          return 4
docs: Add the value found in Step 4 to every element of each covered
          row, and subtract it from every element of each uncovered column.
          Return to Step 4 without altering any stars, primes, or covered
          lines.