code: public static <R> ImmutableGrid<R> of(int rowCount, int columnCount, int row, int column, R value) {
          return new SingletonGrid<R>(rowCount, columnCount, row, column, value);
      }
docs: Obtains an immutable grid of the specified row-column count with a single 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 row  the row of the single cell, zero or greater
  @param column  the column of the single cell, zero or greater
  @param value  the value of the single cell, not null
  @return the empty immutable grid, not null