code: public static <R> ImmutableCell<R> copyOf(Cell<? extends R> cell) {
          if (cell == null) {
              throw new IllegalArgumentException("Cell must not be null");
          }
          if (cell instanceof ImmutableCell) {
              @SuppressWarnings("unchecked")
              ImmutableCell<R> result = (ImmutableCell<R>) cell;
              return result;
          }
          return ImmutableCell.<R>of(cell.getRow(), cell.getColumn(), cell.getValue());
      }
docs: Obtains an instance of {@code Cell}.
  
  @param <R> the type of the value
  @param cell  the cell to copy, not null
  @return the immutable cell, not null