message: refactor(expand): even smaller
diff: @@ -81,7 +81,7 @@ export function expand<T, R>(
  lift(source, function (this: Subscriber<any>, source: Observable<T>) {
  const subscriber = this;
  let active = 0;
  - const buffer: T[] = [];
  + const buffer: (T | R)[] = [];
  let index = 0;
  let isComplete = false;
  @@ -89,13 +89,8 @@ export function expand<T, R>(
  while (0 < buffer.length && active < concurrent) {
  const value = buffer.shift()!;
  subscriber.next(value);
  - let inner: Observable<any>;
  - try {
  - inner = from(project(value, index++));
  - } catch (err) {
  - subscriber.error(err);
  - return;
  - }
  + // TODO: Correct the types here. `project` could be R or T.
  + const inner = from(project(value as any, index++));
  active++;
  const doSub = () => {
  inner.subscribe(
  @@ -108,7 +103,7 @@ export function expand<T, R>(
  }
  };
  - const next = (value: T) => {
  + const next = (value: T | R) => {
  buffer.push(value);
  trySub();
  };
  