code: public static <R> ImmutableGrid<R> copyOf(int rowCount, int columnCount, Cell<R> cell) {
          if (cell == null) {
              throw new IllegalArgumentException("Cell must not be null");
          }
          return new SingletonGrid<R>(rowCount, columnCount, cell);
      }
docs: Obtains an immutable grid with one cell.
  
  @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
  @param cell  the cell that the grid should contain, not null
  @return the immutable grid, not null
  @throws IndexOutOfBoundsException if either index is less than zero