* natural ordering or by a supplied comparator. Note that iterations across the columns keys for a
 * single row key may or may not be ordered, depending on the implementation. When rows and columns
 * are both sorted, it's easier to use the {@link TreeBasedTable} subclass.
 *
 * <p>The {@link #rowKeySet} method returns a {@link SortedSet} and the {@link #rowMap} method