code: private boolean isIntroducingFixedConflict(Sudoku sudoku,
                                                 int row,
                                                 int fromIndex,
                                                 int toIndex)
      {
          return columnFixedValues[fromIndex][sudoku.getValue(row, toIndex) - 1]
                 || columnFixedValues[toIndex][sudoku.getValue(row, fromIndex) - 1]
                 || subGridFixedValues[convertToSubGrid(row, fromIndex)][sudoku.getValue(row, toIndex) - 1]
                 || subGridFixedValues[convertToSubGrid(row, toIndex)][sudoku.getValue(row, fromIndex) - 1];
      }
docs: Checks whether the proposed mutation would introduce a duplicate of a fixed value
  into a column or sub-grid.