@Test
public void shouldSubstractFailOnEmptyOperand() {

  final Object[][] parameters = new Object[][] {
      {Aer.EMPTY, Aer.ONE},
      {Aer.ONE, Aer.EMPTY}
  };
  for (final Object[] parameter : parameters) {
    final Aer left = (Aer) parameter[0];
    final Aer right = (Aer) parameter[1];
    try {
      // when
      left.subtract(right);
      fail();
    } catch (AssertionError e) {
      // then
    }
  }
}