@Test
public void testRetrieveGroup() {

    Optional<Group> result = engineManager.retrieveGroup(ERROR);
    verify(engine).retrieveGroup(ERROR);
    assertFalse(result.isPresent());
    result = engineManager.retrieveGroup("group");
    verify(engine).retrieveGroup("group");
    assertTrue(result.isPresent());
    assertEquals(group, result.get());
}