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+import { OperatorFunction, ObservableInput, ObservedValueOf, SchedulerLike } from '../types';
+import { operate } from '../util/lift';
+import { mergeInternals } from './mergeInternals';
+
+/* tslint:disable:max-line-length */
+export function expand<T, O extends ObservableInput<unknown>>(
+  project: (value: T, index: number) => O,
+  concurrent?: number,
+  scheduler?: SchedulerLike
+): OperatorFunction<T, ObservedValueOf<O>>;
+/**
+ * @deprecated The `scheduler` parameter will be removed in v8. If you need to schedule the inner subscription,
+ * use `subscribeOn` within the projection function: `expand((value) => fn(value).pipe(subscribeOn(scheduler)))`.
+ * Details: Details: https://rxjs.dev/deprecations/scheduler-argument
+ */
+export function expand<T, O extends ObservableInput<unknown>>(
+  project: (value: T, index: number) => O,
+  concurrent: number | undefined,
+  scheduler: SchedulerLike
+): OperatorFunction<T, ObservedValueOf<O>>;
+/* tslint:enable:max-line-length */
+
+/**
+ * Recursively projects each source value to an Observable which is merged in
+ * the output Observable.
+ *
+ * <span class="informal">It's similar to {@link mergeMap}, but applies the
+ * projection function to every source value as well as every output value.
+ * It's recursive.</span>
+ *
+ * ![](expand.png)
+ *
+ * Returns an Observable that emits items based on applying a function that you
+ * supply to each item emitted by the source Observable, where that function
+ * returns an Observable, and then merging those resulting Observables and
+ * emitting the results of this merger. *Expand* will re-emit on the output
+ * Observable every source value. Then, each output value is given to the
+ * `project` function which returns an inner Observable to be merged on the
+ * output Observable. Those output values resulting from the projection are also
+ * given to the `project` function to produce new output values. This is how
+ * *expand* behaves recursively.
+ *
+ * ## Example
+ *
+ * Start emitting the powers of two on every click, at most 10 of them
+ *
+ * ```ts
+ * import { fromEvent, map, expand, of, delay, take } from 'rxjs';
+ *
+ * const clicks = fromEvent(document, 'click');
+ * const powersOfTwo = clicks.pipe(
+ *   map(() => 1),
+ *   expand(x => of(2 * x).pipe(delay(1000))),
+ *   take(10)
+ * );
+ * powersOfTwo.subscribe(x => console.log(x));
+ * ```
+ *
+ * @see {@link mergeMap}
+ * @see {@link mergeScan}
+ *
+ * @param project A function that, when applied to an item emitted by the source
+ * or the output Observable, returns an Observable.
+ * @param concurrent Maximum number of input Observables being subscribed to
+ * concurrently.
+ * @param scheduler The {@link SchedulerLike} to use for subscribing to
+ * each projected inner Observable.
+ * @return A function that returns an Observable that emits the source values
+ * and also result of applying the projection function to each value emitted on
+ * the output Observable and merging the results of the Observables obtained
+ * from this transformation.
+ */
+export function expand<T, O extends ObservableInput<unknown>>(
+  project: (value: T, index: number) => O,
+  concurrent = Infinity,
+  scheduler?: SchedulerLike
+): OperatorFunction<T, ObservedValueOf<O>> {
+  concurrent = (concurrent || 0) < 1 ? Infinity : concurrent;
+  return operate((source, subscriber) =>
+    mergeInternals(
+      // General merge params
+      source,
+      subscriber,
+      project,
+      concurrent,
+
+      // onBeforeNext
+      undefined,
+
+      // Expand-specific
+      true, // Use expand path
+      scheduler // Inner subscription scheduler
+    )
+  );
+}