* support {@link Iterator#remove}. If the table is modified after an iterator is created, the
 * iterator remains valid.
 *
 * <p>This class requires less memory than the {@link HashBasedTable} and {@link TreeBasedTable}
 * implementations, except when the table is sparse.
 *
 * <p>Null row keys or column keys are not permitted.
 *
 * <p>This class provides methods involving the underlying array structure, where the array indices