diff --git a/expensiveOptimized.ts b/expensiveOptimized.ts
new file mode 100644
index 0000000..177cf7b
--- /dev/null
+++ b/expensiveOptimized.ts
@@ -0,0 +1,172 @@
+interface Iterable<C> {}
+
+const unrelated = Symbol()
+type unrelated = typeof unrelated
+interface Collections<K,V> {
+  [unrelated]: V[]
+}
+
+interface ZipConcatMixin<T,SelfIdentifier extends keyof Collections<any,any>> {
+  zip<U>(other: Collection<any, U>): Collections<number, [T,U]>[SelfIdentifier];
+  zip<U,V>(other: Collection<any, U>, other2: Collection<any,V>): Collections<number, [T,U,V]>[SelfIdentifier];
+  //zip<U,V,M>(other: Collection<any, U>, other2: Collection<any,V>, other3: Collection<any,M>): Collections<[T,U,V,M]>[SelfIdentifier];
+  zip(...collections: Array<Collection<any, any>>): Collections<number,any>[SelfIdentifier];
+
+  concat<C>(...valuesOrCollections: Array<Iterable<C> | C>): Collections<number,T | C>[SelfIdentifier];
+}
+
+interface MapFilterMixin<K,V,SelfIdentifier extends keyof Collections<any,any>> {
+  map<M>(
+    mapper: (value: V, key: K, iter: this) => M,
+    context?: any
+  ): Collections<K,M>[SelfIdentifier];
+  // flatMap<M>(
+  //   mapper: (value: V, key: K, iter: this) => Iterable<M>,
+  //   context?: any
+  // ): Collections<K,M>[SelfIdentifier];
+  filter<F extends V>(
+    predicate: (value: V, index: K, iter: this) => value is F,
+    context?: any
+  ): Collections<K,F>[SelfIdentifier];
+  filter(
+    predicate: (value: V, index: K, iter: this) => any,
+    context?: any
+  ): this;
+}
+
+export interface List<T> extends CollectionIndexed<T, ListSelf> {
+}
+const ListSelf = Symbol()
+type ListSelf = typeof ListSelf
+interface Collections<K,V> {
+  [ListSelf]: List<V>
+}
+
+export interface SeqIndexed<T> extends CollectionIndexed<T, SeqIndexedSelf> {
+  toSeq(): this
+
+  zipAll(...collections: Array<Collection<any, any>>): SeqIndexed<any>;
+}
+const SeqIndexedSelf = Symbol()
+type SeqIndexedSelf = typeof SeqIndexedSelf
+interface Collections<K,V> {
+  [SeqIndexedSelf]: SeqIndexed<V>
+}
+
+export interface CollectionIndexed<T, SelfIdentifier extends keyof Collections<any, any> = CollectionIndexedSelf> extends Collection<number, T, SelfIdentifier>, ZipConcatMixin<T, SelfIdentifier> {
+  interpose(separator: T): this;
+}
+const CollectionIndexedSelf = Symbol()
+type CollectionIndexedSelf = typeof CollectionIndexedSelf
+interface Collections<K,V> {
+  [CollectionIndexedSelf]: CollectionIndexed<V>
+}
+
+export interface Collection<K, V, SelfIdentifier extends keyof Collections<any,any> = CollectionSelf> extends MapFilterMixin<K,V,SelfIdentifier> {
+
+  // Conversion to JavaScript types
+
+  toJS(): Array<any> | { [key: string]: any };
+  toJSON(): Array<V> | { [key: string]: V };
+
+  // Conversion to Collections
+
+  toMap(): Collection<K, V>;
+  toOrderedMap(): Collection<K, V>;
+  toSet(): Collection<V,V>;
+  toOrderedSet(): Collection<V,V>;
+  toList(): List<V>;
+  toStack(): Collection<V,V>;
+
+  // Conversion to Seq
+
+  toSeq(): Collection<K, V>;
+  toKeyedSeq(): Collection<K, V>;
+  toIndexedSeq(): SeqIndexed<V>;
+
+  // Sequence algorithms
+
+  // map<M>(
+  //   mapper: (value: V, key: K, iter: this) => M,
+  //   context?: any
+  // ): Collection<K, M>;
+  // filter<F extends V>(
+  //   predicate: (value: V, key: K, iter: this) => value is F,
+  //   context?: any
+  // ): Collection<K, F>;
+  // filter(
+  //   predicate: (value: V, key: K, iter: this) => any,
+  //   context?: any
+  // ): this;
+  filterNot(
+    predicate: (value: V, key: K, iter: this) => boolean,
+    context?: any
+  ): this;
+  sort(comparator?: (valueA: V, valueB: V) => number): this;
+  sortBy<C>(
+    comparatorValueMapper: (value: V, key: K, iter: this) => C,
+    comparator?: (valueA: C, valueB: C) => number
+  ): this;
+  groupBy<G>(
+    grouper: (value: V, key: K, iter: this) => G,
+    context?: any
+  ): /*Map*/Collection<G, /*this*/Collection<K, V>>;
+
+  // Padding
+  takeUntil0(predicate: (iter: this) => boolean): this;
+  takeUntil1(predicate: (iter: this) => boolean): this;
+  takeUntil2(predicate: (iter: this) => boolean): this;
+  takeUntil3(predicate: (iter: this) => boolean): this;
+  takeUntil4(predicate: (iter: this) => boolean): this;
+  takeUntil5(predicate: (iter: this) => boolean): this;
+  takeUntil6(predicate: (iter: this) => boolean): this;
+  ttakeUntil0(predicate: (iter: this) => boolean): this;
+  ttakeUntil1(predicate: (iter: this) => boolean): this;
+  ttakeUntil2(predicate: (iter: this) => boolean): this;
+  ttakeUntil3(predicate: (iter: this) => boolean): this;
+  ttakeUntil4(predicate: (iter: this) => boolean): this;
+  ttakeUntil5(predicate: (iter: this) => boolean): this;
+  ttakeUntil6(predicate: (iter: this) => boolean): this;
+  tttakeUntil0(predicate: (iter: this) => boolean): this;
+  tttakeUntil1(predicate: (iter: this) => boolean): this;
+  tttakeUntil2(predicate: (iter: this) => boolean): this;
+  tttakeUntil3(predicate: (iter: this) => boolean): this;
+  tttakeUntil4(predicate: (iter: this) => boolean): this;
+  tttakeUntil5(predicate: (iter: this) => boolean): this;
+  tttakeUntil6(predicate: (iter: this) => boolean): this;
+  ttttakeUntil0(predicate: (iter: this) => boolean): this;
+  ttttakeUntil1(predicate: (iter: this) => boolean): this;
+  ttttakeUntil2(predicate: (iter: this) => boolean): this;
+  ttttakeUntil3(predicate: (iter: this) => boolean): this;
+  ttttakeUntil4(predicate: (iter: this) => boolean): this;
+  ttttakeUntil5(predicate: (iter: this) => boolean): this;
+  ttttakeUntil6(predicate: (iter: this) => boolean): this;
+
+  tttttakeUntil0(predicate: (iter: this) => boolean): this;
+  tttttakeUntil1(predicate: (iter: this) => boolean): this;
+  tttttakeUntil2(predicate: (iter: this) => boolean): this;
+  tttttakeUntil3(predicate: (iter: this) => boolean): this;
+  tttttakeUntil4(predicate: (iter: this) => boolean): this;
+  tttttakeUntil5(predicate: (iter: this) => boolean): this;
+  tttttakeUntil6(predicate: (iter: this) => boolean): this;
+}
+const CollectionSelf = Symbol()
+type CollectionSelf = typeof CollectionSelf
+interface Collections<K,V> {
+  [CollectionSelf]: Collection<K,V>
+}
+
+function a<T> () {
+  const a: SeqIndexed<T> = 0 as any
+  const b: CollectionIndexed<T> = a
+
+  const c: CollectionIndexed<T, SeqIndexedSelf> = 0 as any
+  c.zip(0 as any as Collection<any, T>)
+  const d: CollectionIndexed<T, CollectionIndexedSelf> = c
+  d.zip(0 as any as Collection<any, T>)
+
+  return b
+}
+
+const b: SeqIndexed<(number | string)> = 0 as any
+const c = b.filter((v): v is number => typeof v !== "string")
\ No newline at end of file