@Test
public void testFoldDoubleByColumn() {

    DataFrame df = DataFrameBuilder
            .builder("a", "b")
            .foldDoubleByColumn(-9999.9, 0, 1.1, 2, 3, 4, 5);
    new DataFrameAsserts(df, "a", "b").expectHeight(3)
            .expectDoubleColumns(0, 1)
            .expectRow(0, 0., 3.)
            .expectRow(1, 1.1, 4.)
            .expectRow(2, 2., 5.);
}