code: public static <R> SparseGrid<R> create(int rowCount, int columnCount) {
          return new SparseGrid<R>(rowCount, columnCount, new TreeSet<Cell<R>>(AbstractCell.<R>comparator()));
      }
docs: Creates an empty {@code SparseGrid} of the specified row-column count.
  
  @param <R> the type of the value
  @param rowCount  the number of rows, zero or greater
  @param columnCount  the number of columns, zero or greater
  @return the mutable grid, not null