return function(this: TrackableSubCollection<T, O, U, C>, idOrIds?: string | string[]) {
						if (idOrIds || !instanceStateMap.get(this).isTracking) {
							return observe.call(this, idOrIds);
						}
						else {
							return instanceStateMap.get(this).observable;
						}
					};

			updateAndAddIds.forEach(function(id, itemIndex) {
				if (newIndex.has(id) && !state.idToIndex.has(id)) {
					if (itemIndex < update.updates.length) {
						trackedUpdates.push(update.updates[itemIndex]);
					}
					else {
						trackedAdds.push(update.adds[itemIndex - update.updates.length]);
					}
					trackedAdds.push();
					const index = newIndex.get(id);
					addedToTracked.push({
						item: newData[index],
						id: id,
						index: index
					});
				}
			});

			updateIds.forEach(function(id, updateIndex) {
				if (!newIndex.has(id) && state.idToIndex.has(id)) {
					trackedUpdates.push(update.updates[updateIndex]);
					const index = state.idToIndex.get(id);
					removedFromTracked.push({
						item: update.updates[updateIndex],
						previousIndex: index,
						id: id
					});
				}
				else if (newIndex.has(id) && state.idToIndex.has(id)) {
					trackedUpdates.push(update.updates[updateIndex]);
					const previouxIndex = state.idToIndex.get(id);
					const index = newIndex.get(id);
					movedInTracked.push({
						item: newData[index],
						id: id,
						previousIndex: previouxIndex,
						index: index
					});
				}
			});

const idToWidgetMap = new Map<string, TodoItem>();
const widgetToIdMap = new WeakMap<TodoItem, string>();

interface TodoRegistryOptions {
	widgetStore: MemoryStore<Object>;
}

interface TodoRegistry {
	get(id: string): Promise<TodoItem>;
	identify(value: TodoItem): string;
	widgetStore?: MemoryStore<Object>;
}

const todoRegistryFactory = compose({
	get(id: string): Promise<TodoItem> {
		let widget: TodoItem = idToWidgetMap.get(id);
		if (!widget) {
			widget = createTodoItem({id, stateFrom: this.widgetStore});
			widgetToIdMap.set(widget, id);
			idToWidgetMap.set(id, widget);
		}
		return Promise.resolve(widget);
	},
	identify(value: TodoItem): string {
		return widgetToIdMap.get(value);
	}
}, function (todoRegistry: TodoRegistry, options: TodoRegistryOptions) {
	if (options) {
		todoRegistry.widgetStore = options.widgetStore;
	}
});

					const createSigned = (): [ Record, Record ] => {
						return [
							// Record negative and positive columns separately.
							{ originalPoints: [], columns: [], isNegative: true, relativeValue: 0, value: 0 },
							{ originalPoints: [], columns: [], isNegative: false, relativeValue: 0, value: 0 }
						];
					};

					for (const point of originalPoints) {
						const { datum } = point;
						const { input, relativeValue, value } = datum;

						// Note that the ordering of the stacks is determined by the original sort order, as is the
						// ordering of nodes within the stack.
						const stack = stackSelector(input);
						const signed = stacks.get(stack) || createSigned();
						const record = relativeValue < 0 ? signed[0] : signed[1];
						if (!stacks.has(stack)) {
							stacks.set(stack, signed);
						}

						record.originalPoints.push(point);
						record.columns.push(datum);
						record.relativeValue += relativeValue;
						record.value += value;

						if (record.relativeValue < mostNegativeRelValue) {
							mostNegativeRelValue = record.relativeValue;
						}
						else if (record.relativeValue > mostPositiveRelValue) {
							mostPositiveRelValue = record.relativeValue;

				assert.doesNotThrow(function () {
					const method = map.get('method');
					method && method();
				});

	'with maps': {

		beforeEach() {
			methodSpy = sinon.spy(function (a: number) {
				return a + 1;
			});
			map = new Map([ [ 'method', methodSpy ] ]);
		},

		'.before': {
			'return value passed as arguments'() {
				let aspectSpy = createBeforeSpy();

				aspect.before(map, 'method', aspectSpy);

				const method = map.get('method');
				method && method(0);
				assert.isTrue(aspectSpy.calledBefore(methodSpy));
				assert.isTrue(aspectSpy.calledOnce);
				assert.isTrue(methodSpy.calledOnce);
				assert.strictEqual(aspectSpy.lastCall.args[0], 0);
				assert.strictEqual(methodSpy.lastCall.args[0], 1);
				assert.strictEqual(methodSpy.returnValues[0], 2);
			},

			'no return value from advising function'() {
				let receivedArgs: string[] = [];
				let beforeCalled = false;
				map = new Map([ [ 'method', function (...args: string[]) {
						receivedArgs = args;
					} ] ]);

						totalValue: number;
						value: number;
						y1: number;
					}
					const groups = new Map<G, Record>();
					const createRecord = (): Record => {
						return { originalPoints: [], columns: [], totalValue: 0, value: 0, y1: columnHeight };
					};

					for (const point of originalPoints) {
						const { input, relativeValue, value } = point.datum;

						// Note that the ordering of the groups is determined by the original sort order, as is the
						// ordering of nodes within the group.
						const group = groupSelector(input);
						const record = groups.get(group) || createRecord();
						if (!groups.has(group)) {
							groups.set(group, record);
						}

						record.originalPoints.push(point);
						record.columns.push(point.datum);
						record.totalValue += value;
						if (relativeValue < 0) {
							record.value = Math.min(record.value, value);
						}
						else {
							// Note that the expected value for mixed groups is undefined.
							record.value = Math.max(record.value, value);
						}
						record.y1 = Math.min(record.y1, point.y1);