code: static <R, C, V> Cell<R, C, V> cellOf(R rowKey, C columnKey, V value) {
      return Tables.immutableCell(checkNotNull(rowKey), checkNotNull(columnKey), checkNotNull(value));
    }
docs: Verifies that {@code rowKey}, {@code columnKey} and {@code value} are
  non-null, and returns a new entry with those values.