public static <T> List<LocalProperty<T>> grouped(Collection<T> columns)
    {
        return ImmutableList.of(new GroupingProperty<>(columns));
    }
@Test
    public void testNormalizeSingleSymbolGroup()
    {
        List<LocalProperty<String>> localProperties = builder().grouped("a").build();
        assertNormalize(localProperties, Optional.of(grouped("a")));
        assertNormalizeAndFlatten(localProperties, grouped("a"));
    }