@Test
public void testFoldIntByColumn() {

    DataFrame df = DataFrameBuilder
            .builder("a", "b")
            .foldIntByColumn(-9999, 0, 1, 2, 3, 4, 5);
    new DataFrameAsserts(df, "a", "b").expectHeight(3)
            .expectIntColumns(0, 1)
            .expectRow(0, 0, 3)
            .expectRow(1, 1, 4)
            .expectRow(2, 2, 5);
}