code: public void makeReferences(CellRow row, CellCol col, CellSqr sqr) {
          this.cellRow = row;
          this.cellCol = col;
          this.cellSqr = sqr;
          this.exCells = new HashSet<Cell>();
          this.exCells.addAll(this.cellRow.getCells());
          this.exCells.addAll(this.cellCol.getCells());
          this.exCells.addAll(this.cellSqr.getCells());
          this.exCells.remove(this);
      }
docs: Set references to all cell groups containing this cell.
  @param row the cell group for the row
  @param col the cell group for the column
  @param sqr the cell group for the square 3x3 area