@Test
public void testGroup_Agg() {

    DataFrame df1 = DataFrame.newFrame("a", "b").foldByRow(
            1, "x",
            2, "y",
            1, "z",
            0, "a",
            1, "x");
    DataFrame df = df1.group("a").agg(Aggregator.sumLong("a"), Aggregator.concat("b", ";"));
    new DataFrameAsserts(df, "a", "b")
            .expectHeight(3)
            .expectRow(0, 3L, "x;z;x")
            .expectRow(1, 2L, "y")
            .expectRow(2, 0L, "a");
}