@Test
public void testFoldIntStreamByColumn() {

    DataFrame df = DataFrameBuilder
            .builder("a", "b")
            .foldIntStreamByColumn(-9999, IntStream.of(-1, 1, 0, 2, 5, 3));
    new DataFrameAsserts(df, "a", "b").expectHeight(3)
            .expectIntColumns(0, 1)
            .expectRow(0, -1, 2)
            .expectRow(1, 1, 5)
            .expectRow(2, 0, 3);
}