code: public static <R> SparseGrid<R> create(Grid<? extends R> grid) {
          if (grid == null) {
              throw new IllegalArgumentException("Grid must not be null");
          }
          SparseGrid<R> created = SparseGrid.create(grid.rowCount(), grid.columnCount());
          created.putAll(grid);
          return created;
      }
docs: Creates a {@code SparseGrid} copying from another grid.
  
  @param <R> the type of the value
  @param grid  the grid to copy, not null
  @return the mutable grid, not null