@Test
public void testCompareTo() {

  final Aer bottom = Aer.of("1", Unit.AER);
  final Aer middle = Aer.of("2", Unit.AER);
  final Aer top = Aer.of("3", Unit.AER);
  assertEquals(1, middle.compareTo(bottom));
  assertEquals(0, middle.compareTo(middle));
  assertEquals(-1, middle.compareTo(top));
}