* Tests the append method of the DfsReferral class.
     * It verifies that a new DfsReferral object can be successfully appended to an existing one,
     * forming a linked list structure.
     */
    @Test
    void testAppend() {
        // Given
        DfsReferral initialReferral = new DfsReferral();
        DfsReferral appendedReferral = new DfsReferral();

        // When
        initialReferral.append(appendedReferral);

            assertNotNull(concreteResult);
        }

        @Test
        @DisplayName("Should append referral data")
        void testAppend() {
            DfsReferralDataInternal toAppend = mock(DfsReferralDataInternal.class);

            // Test with mock
            doNothing().when(mockReferralData).append(toAppend);

        }
    }

    @Nested
    @DisplayName("Linked List Operations Tests")
    class LinkedListTests {

        @Test
        @DisplayName("Should append referral data")
        void testAppend() {
            DfsReferralDataImpl second = new DfsReferralDataImpl();
            DfsReferralDataImpl third = new DfsReferralDataImpl();

            referralData.append(second);

    iterator.next();
    iterator.remove();
    iterator.next();
    iterator.remove();
    assertFalse(iterator.hasNext());
    assertFalse(cycle.iterator().hasNext());
  }

  public void testAppend() {
    FluentIterable<Integer> result =
        FluentIterable.<Integer>from(asList(1, 2, 3)).append(Lists.newArrayList(4, 5, 6));
    assertEquals(asList(1, 2, 3, 4, 5, 6), Lists.newArrayList(result));