diff --git a/src/extra/flattenConcurrently.ts b/src/extra/flattenConcurrently.ts
index bb358ed..9afd51c 100644
--- a/src/extra/flattenConcurrently.ts
+++ b/src/extra/flattenConcurrently.ts
@@ -1,4 +1,4 @@
-import {Operator, Stream, OutSender, InternalListener} from '../index';
+import { Operator, Stream, MemoryStream, OutSender, InternalListener } from '../index';
 
 class FCIL<T> implements InternalListener<T>, OutSender<T> {
   constructor(public out: Stream<T>,
@@ -88,6 +88,6 @@ export class FlattenConcOperator<T> implements Operator<Stream<T>, T> {
  *
  * @return {Stream}
  */
-export default function flattenConcurrently<T>(ins: Stream<Stream<T>>): Stream<T> {
+export default function flattenConcurrently<T>(ins: Stream<Stream<T> | MemoryStream<T>>): Stream<T> {
   return new Stream<T>(new FlattenConcOperator(ins));
 }
diff --git a/src/extra/flattenConcurrentlyAtMost.ts b/src/extra/flattenConcurrentlyAtMost.ts
index eb34103..57033ad 100644
--- a/src/extra/flattenConcurrentlyAtMost.ts
+++ b/src/extra/flattenConcurrentlyAtMost.ts
@@ -1,4 +1,4 @@
-import {Operator, Stream, OutSender, InternalListener} from '../index';
+import { Operator, Stream, MemoryStream, OutSender, InternalListener } from '../index';
 
 class FCAMIL<T> implements InternalListener<T>, OutSender<T> {
   constructor(public out: Stream<T>,
@@ -125,8 +125,8 @@ export class FlattenConcAMOperator<T> implements Operator<Stream<T>, T> {
  *
  * @return {Stream}
  */
-export default function flattenConcurrentlyAtMost<T>(n: number): (ins: Stream<Stream<T>>) => Stream<T> {
-  return function flattenConcAMOperator(ins: Stream<Stream<T>>) {
+export default function flattenConcurrentlyAtMost<T>(n: number): (ins: Stream<Stream<T> | MemoryStream<T>>) => Stream<T> {
+  return function flattenConcAMOperator(ins: Stream<Stream<T> | MemoryStream<T>>) {
     return new Stream<T>(new FlattenConcAMOperator(n, ins));
   };
 }
diff --git a/src/extra/flattenSequentially.ts b/src/extra/flattenSequentially.ts
index bcb9abc..55986bd 100644
--- a/src/extra/flattenSequentially.ts
+++ b/src/extra/flattenSequentially.ts
@@ -1,4 +1,4 @@
-import {Operator, Stream, InternalListener} from '../index';
+import { Operator, Stream, MemoryStream, InternalListener } from '../index';
 
 class FSInner<T> implements InternalListener<T> {
   constructor(private out: Stream<T>,
@@ -122,6 +122,6 @@ export class FlattenSeqOperator<T> implements Operator<Stream<T>, T> {
  *
  * @return {Stream}
  */
-export default function flattenSequentially<T>(ins: Stream<Stream<T>>): Stream<T> {
+export default function flattenSequentially<T>(ins: Stream<Stream<T> | MemoryStream<T>>): Stream<T> {
   return new Stream<T>(new FlattenSeqOperator(ins));
 }
diff --git a/src/index.ts b/src/index.ts
index 2147813..04deffd 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,7 +1,7 @@
 import $$observable from 'symbol-observable';
 
 const NO = {};
-function noop() {}
+function noop() { }
 
 function cp<T>(a: Array<T>): Array<T> {
   const l = a.length;
@@ -93,7 +93,7 @@ function internalizeProducer<T>(producer: Producer<T> & Partial<InternalProducer
 }
 
 class StreamSub<T> implements Subscription {
-  constructor(private _stream: Stream<T>, private _listener: InternalListener<T>) {}
+  constructor(private _stream: Stream<T>, private _listener: InternalListener<T>) { }
 
   unsubscribe(): void {
     this._stream._remove(this._listener);
@@ -101,7 +101,7 @@ class StreamSub<T> implements Subscription {
 }
 
 class Observer<T> implements Listener<T> {
-  constructor(private _listener: InternalListener<T>) {}
+  constructor(private _listener: InternalListener<T>) { }
 
   next(value: T) {
     this._listener._n(value);
@@ -772,7 +772,7 @@ class Flatten<T> implements Operator<Stream<T>, T> {
   _n(s: Stream<T>) {
     const u = this.out;
     if (u === NO) return;
-    const {inner, il} = this;
+    const { inner, il } = this;
     if (inner !== NO && il !== NO_IL) inner._remove(il);
     (this.inner = s)._add(this.il = new FlattenListener(u, this));
   }
@@ -1182,16 +1182,16 @@ export class Stream<T> implements InternalListener<T> {
   _hasNoSinks(x: InternalListener<any>, trace: Array<any>): boolean {
     if (trace.indexOf(x) !== -1)
       return true; else
-    if ((x as any as OutSender<any>).out === this)
-      return true; else
-    if ((x as any as OutSender<any>).out && (x as any as OutSender<any>).out !== NO)
-      return this._hasNoSinks((x as any as OutSender<any>).out, trace.concat(x)); else
-    if ((x as Stream<any>)._ils) {
-      for (let i = 0, N = (x as Stream<any>)._ils.length; i < N; i++)
-        if (!this._hasNoSinks((x as Stream<any>)._ils[i], trace.concat(x)))
-          return false;
-      return true;
-    } else return false;
+      if ((x as any as OutSender<any>).out === this)
+        return true; else
+        if ((x as any as OutSender<any>).out && (x as any as OutSender<any>).out !== NO)
+          return this._hasNoSinks((x as any as OutSender<any>).out, trace.concat(x)); else
+          if ((x as Stream<any>)._ils) {
+            for (let i = 0, N = (x as Stream<any>)._ils.length; i < N; i++)
+              if (!this._hasNoSinks((x as Stream<any>)._ils[i], trace.concat(x)))
+                return false;
+            return true;
+          } else return false;
   }
 
   private ctor(): typeof Stream {
@@ -1251,7 +1251,7 @@ export class Stream<T> implements InternalListener<T> {
   static create<T>(producer?: Producer<T>): Stream<T> {
     if (producer) {
       if (typeof producer.start !== 'function'
-      || typeof producer.stop !== 'function')
+        || typeof producer.stop !== 'function')
         throw new Error('producer requires both start and stop functions');
       internalizeProducer(producer); // mutates the input
     }
@@ -1285,7 +1285,7 @@ export class Stream<T> implements InternalListener<T> {
    * @return {Stream}
    */
   static never(): Stream<any> {
-    return new Stream<any>({_start: noop, _stop: noop});
+    return new Stream<any>({ _start: noop, _stop: noop });
   }
 
   /**
@@ -1342,10 +1342,10 @@ export class Stream<T> implements InternalListener<T> {
   static from<T>(input: PromiseLike<T> | Stream<T> | Array<T> | Observable<T>): Stream<T> {
     if (typeof input[$$observable] === 'function')
       return Stream.fromObservable<T>(input as Observable<T>); else
-    if (typeof (input as PromiseLike<T>).then === 'function')
-      return Stream.fromPromise<T>(input as PromiseLike<T>); else
-    if (Array.isArray(input))
-      return Stream.fromArray<T>(input);
+      if (typeof (input as PromiseLike<T>).then === 'function')
+        return Stream.fromPromise<T>(input as PromiseLike<T>); else
+        if (Array.isArray(input))
+          return Stream.fromArray<T>(input);
 
     throw new TypeError(`Type of input to from() must be an Array, Promise, or Observable`);
   }
@@ -1417,7 +1417,7 @@ export class Stream<T> implements InternalListener<T> {
    * @param {any} observable The observable to be converted as a stream.
    * @return {Stream}
    */
-  static fromObservable<T>(obs: {subscribe: any}): Stream<T> {
+  static fromObservable<T>(obs: { subscribe: any }): Stream<T> {
     if ((obs as Stream<T>).endWhen) return obs as Stream<T>;
     const o = typeof obs[$$observable] === 'function' ? obs[$$observable]() : obs;
     return new Stream<T>(new FromObservable(o));
@@ -1765,7 +1765,7 @@ export class Stream<T> implements InternalListener<T> {
    *
    * @return {Stream}
    */
-  flatten<R>(this: Stream<Stream<R>>): Stream<R> {
+  flatten<R>(this: Stream<Stream<R> | MemoryStream<R>>): Stream<R> {
     return new Stream<R>(new Flatten(this));
   }
 
@@ -1895,8 +1895,8 @@ export class Stream<T> implements InternalListener<T> {
   imitate(target: Stream<T>): void {
     if (target instanceof MemoryStream)
       throw new Error('A MemoryStream was given to imitate(), but it only ' +
-      'supports a Stream. Read more about this restriction here: ' +
-      'https://github.com/staltz/xstream#faq');
+        'supports a Stream. Read more about this restriction here: ' +
+        'https://github.com/staltz/xstream#faq');
     this._target = target;
     for (let ils = this._ils, N = ils.length, i = 0; i < N; i++) target._add(ils[i]);
     this._ils = [];
@@ -2049,7 +2049,7 @@ export class MemoryStream<T> extends Stream<T> {
   }
 }
 
-export {NO, NO_IL};
+export { NO, NO_IL };
 const xs = Stream;
 type xs<T> = Stream<T>;
 export default xs;