In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

    assertEquals("Cause should be available.", exception, handler.exception);
    assertEquals("EventBus should be available.", eventBus, handler.context.getEventBus());
    assertEquals("Event should be available.", EVENT, handler.context.getEvent());
    assertEquals("Subscriber should be available.", subscriber, handler.context.getSubscriber());
    assertEquals(
        "Method should be available.",

    assertEquals("Cause should be available.", exception, handler.exception);
    assertEquals("EventBus should be available.", eventBus, handler.context.getEventBus());
    assertEquals("Event should be available.", EVENT, handler.context.getEvent());
    assertEquals("Subscriber should be available.", subscriber, handler.context.getSubscriber());
    assertEquals(
        "Method should be available.",

	if err != nil {
		t.Fatalf("Should open db success, but got %v", err)
	}

	tx := db.Begin()
	user := *GetUser("transaction", Config{})

	if err = tx.Save(&user).Error; err != nil {
		t.Fatalf("No error should raise, but got %v", err)
	}

	if err = tx.First(&User{}, "name = ?", "transaction").Error; err != nil {
		t.Fatalf("Should find saved record, but got %v", err)
	}

    int index = aValidIndex();
    E initialValue = getList().get(index);
    assertEquals(
        "set(i, x) should return the old element at position i.",
        initialValue,
        getList().set(index, newValue));
    assertEquals("After set(i, x), get(i) should return x", newValue, getList().get(index));
    assertEquals("set() should not change the size of a list.", getNumElements(), getList().size());
  }

  @ListFeature.Require(SUPPORTS_SET)

    assertEquals("Should be able to peek() at first element", "A", peekingIterator.peek());
    assertEquals(
        "Should be able to peek() first element multiple times", "A", peekingIterator.peek());
    assertEquals(
        "next() should still return first element after peeking", "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element", "B", peekingIterator.peek());

    assertEquals("Should be able to peek() at first element", "A", peekingIterator.peek());
    assertEquals(
        "Should be able to peek() first element multiple times", "A", peekingIterator.peek());
    assertEquals(
        "next() should still return first element after peeking", "A", peekingIterator.next());

    assertEquals("Should be able to peek() at middle element", "B", peekingIterator.peek());

  @CollectionSize.Require(absent = ZERO)
  public void testRemove_present() {
    int initialSize = collection.size();
    assertTrue("remove(present) should return true", collection.remove(e0()));
    assertEquals(
        "remove(present) should decrease a collection's size by one.",
        initialSize - 1,
        collection.size());
    expectMissing(e0());
  }

    assertSame(
        "toArray(sameSizeE[]) should return the given array", array, collection.toArray(array));
    expectArrayContentsAnyOrder(createSamplesArray(), array);
  }

  @CollectionFeature.Require(KNOWN_ORDER)
  public void testToArray_rightSizedArray_ordered() {
    E[] array = getSubjectGenerator().createArray(getNumElements());
    assertSame(

    assertSame(
        "toArray(sameSizeE[]) should return the given array", array, collection.toArray(array));
    expectArrayContentsAnyOrder(createSamplesArray(), array);
  }

  @CollectionFeature.Require(KNOWN_ORDER)
  public void testToArray_rightSizedArray_ordered() {
    E[] array = getSubjectGenerator().createArray(getNumElements());
    assertSame(