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.",

		t.Error("RowsAffected should be correct when do batch update")
	}

	animal = Animal{From: "somewhere"}              // No name fields, should be filled with the default value (galeone)
	DB.Save(&animal).Update("From", "a nice place") // The name field shoul be untouched
	DB.First(&animal, animal.Counter)
	if animal.Name != "galeone" {
		t.Errorf("Name fields shouldn't be changed if untouched, but got %v", animal.Name)
	}

		t.Fatalf("Should find 1 tags after Delete")
	}

	if DB.Model(&blog).Association("Tags").Count() != 1 {
		t.Fatalf("Blog should has three tags after Delete")
	}

	DB.Model(&blog).Association("Tags").Delete(tag3)
	var tags4 []Tag
	DB.Model(&blog).Association("Tags").Find(&tags4)
	if !compareTags(tags4, []string{"tag6"}) {
		t.Fatalf("Tag should not be deleted when Delete with a unrelated tag")

		t.Errorf("Hamster's preferred toy count should be 1")
	}

	if DB.Model(&hamster2).Association("OtherToy").Count() != 1 {
		t.Errorf("Hamster's other toy count should be 1")
	}

	// Query
	hamsterToy := Toy{}
	DB.Model(&hamster).Association("PreferredToy").Find(&hamsterToy)
	if hamsterToy.Name != hamster.PreferredToy.Name {
		t.Errorf("Should find has one polymorphic association")
	}

	hamsterToy = Toy{}

    ), "overridden configs should be preserved"
    assert (
        '"deepLinking": true' not in response.text
    ), "overridden configs should not include the old value"
    assert (
        '"syntaxHighlight": false' not in response.text
    ), "not used parameters should not be included"
    assert (
        '"dom_id": "#swagger-ui"' in response.text
    ), "default configs should be preserved"

      fail("addAll(n, allPresent) should throw");
    } catch (UnsupportedOperationException expected) {
    }
    expectUnchanged();
  }

  @ListFeature.Require(SUPPORTS_ADD_WITH_INDEX)
  @CollectionSize.Require(absent = ZERO)
  public void testAddAllAtIndex_supportedSomePresent() {
    assertTrue(
        "addAll(n, allPresent) should return true",

  @CollectionSize.Require(absent = ZERO)
  public void testAddAll_supportedSomePresent() {
    assertTrue(
        "addAll(somePresent) should return true",
        collection.addAll(MinimalCollection.of(e3(), e0())));
    assertTrue("should contain " + e3(), collection.contains(e3()));
    assertTrue("should contain " + e0(), collection.contains(e0()));
  }

  @CollectionFeature.Require(absent = SUPPORTS_ADD)

    assertTrue("containsValue(present) should return true", getMap().containsValue(v0()));
  }

  public void testContains_no() {
    assertFalse("containsValue(notPresent) should return false", getMap().containsValue(v3()));
  }

  @MapFeature.Require(ALLOWS_NULL_VALUE_QUERIES)
  public void testContains_nullNotContainedButAllowed() {
    assertFalse("containsValue(null) should return false", getMap().containsValue(null));
  }