@Test
void mapDouble() {

    final ToDouble<Double> toDouble = d -> d;
    final ToDoubleMapper<Double, ToDouble<Double>, DoubleUnaryOperator> toDoubleMapper =
            (ToDoubleMapper<Double, ToDouble<Double>, DoubleUnaryOperator>)
                    MapperUtil.mapDouble(toDouble, d -> d);
    assertEquals(MapperType.DOUBLE_TO_DOUBLE, toDoubleMapper.mapperType());
}