diff --git a/src/index.ts b/src/index.ts
index e69de29..4e20076 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -0,0 +1 @@
+export {sequence} from "./sequence.js";
diff --git a/src/sequence.ts b/src/sequence.ts
new file mode 100644
index 0000000..3a147bf
--- /dev/null
+++ b/src/sequence.ts
@@ -0,0 +1,437 @@
+import {map} from "./sequence/map.js";
+import {collect} from "./sequence/collect.js";
+import {forEach} from "./sequence/forEach.js";
+import {filter} from "./sequence/filter.js";
+import {count} from "./sequence/count.js";
+import {isEmpty} from "./sequence/isEmpty.js";
+import {isNotEmpty} from "./sequence/isNotEmpty.js";
+import {indices} from "./sequence/indices.js";
+import {withIndex} from "./sequence/withIndex.js";
+import {filterIndexed} from "./sequence/filterIndexed.js";
+import {mapIndexed} from "./sequence/mapIndexed.js";
+import {any} from "./sequence/any.js";
+import {none} from "./sequence/none.js";
+import {all} from "./sequence/all.js";
+import {first} from "./sequence/first.js";
+import {firstOrNull} from "./sequence/firstOrNull.js";
+import {last} from "./sequence/last.js";
+import {lastOrNull} from "./sequence/lastOrNull.js";
+import {single} from "./sequence/single.js";
+import {singleOrNull} from "./sequence/singleOrNull.js";
+import {plus} from "./sequence/plus.js";
+import {distinct} from "./sequence/distinct.js";
+import {distinctBy} from "./sequence/distinctBy.js";
+import {drop} from "./sequence/drop.js";
+import {take} from "./sequence/take.js";
+import {slice} from "./sequence/slice.js";
+import {dropWhile} from "./sequence/dropWhile.js";
+import {dropUntil} from "./sequence/dropUntil.js";
+import {takeWhile} from "./sequence/takeWhile.js";
+import {takeUntil} from "./sequence/takeUntil.js";
+import {flatMap} from "./sequence/flatMap.js";
+import {flatMapIndexed} from "./sequence/flatMapIndexed.js";
+import {associate} from "./sequence/associate.js";
+import {associateBy} from "./sequence/associateBy.js";
+import {associateWith} from "./sequence/associateWith.js";
+import {get} from "./sequence/get.js";
+import {getOrNull} from "./sequence/getOrNull.js";
+import {sumBy} from "./sequence/sumBy.js";
+import {averageBy} from "./sequence/averageBy.js";
+import {contains} from "./sequence/contains.js";
+import {containsAll} from "./sequence/containsAll.js";
+import {minBy} from "./sequence/minBy.js";
+import {minByOrNull} from "./sequence/minByOrNull.js";
+import {maxBy} from "./sequence/maxBy.js";
+import {maxByOrNull} from "./sequence/maxByOrNull.js";
+import {minOf} from "./sequence/minOf.js";
+import {minOfOrNull} from "./sequence/minOfOrNull.js";
+import {maxOf} from "./sequence/maxOf.js";
+import {maxOfOrNull} from "./sequence/maxOfOrNull.js";
+import {minWith} from "./sequence/minWith.js";
+import {minWithOrNull} from "./sequence/minWithOrNull.js";
+import {maxWith} from "./sequence/maxWith.js";
+import {maxWithOrNull} from "./sequence/maxWithOrNull.js";
+import {minus} from "./sequence/minus.js";
+import {reduce} from "./sequence/reduce.js";
+import {reduceOrNull} from "./sequence/reduceOrNull.js";
+import {fold} from "./sequence/fold.js";
+import {scan} from "./sequence/scan.js";
+import {partition} from "./sequence/partition.js";
+import {firstIndex} from "./sequence/firstIndex.js";
+import {firstIndexOrNull} from "./sequence/firstIndexOrNull.js";
+import {lastIndex} from "./sequence/lastIndex.js";
+import {lastIndexOrNull} from "./sequence/lastIndexOrNull.js";
+import {sortedWith} from "./sequence/sortedWith.js";
+import {sortedBy} from "./sequence/sortedBy.js";
+import {reversed} from "./sequence/reversed.js";
+import {windowed} from "./sequence/windowed.js";
+import {chunked} from "./sequence/chunked.js";
+import {zip} from "./sequence/zip.js";
+import {unzip} from "./sequence/unzip.js";
+import {sum} from "./sequence/sum.js";
+import {average} from "./sequence/average.js";
+import {min} from "./sequence/min.js";
+import {minOrNull} from "./sequence/minOrNull.js";
+import {max} from "./sequence/max.js";
+import {maxOrNull} from "./sequence/maxOrNull.js";
+import {flatten} from "./sequence/flatten.js";
+import {sorted} from "./sequence/sorted.js";
+import {onEach} from "./sequence/onEach.js";
+import {joinToString} from "./sequence/joinToString.js";
+import {groupBy} from "./sequence/groupBy.js";
+
+export type Sequence<T> = {
+    [Symbol.iterator](): Iterator<T>
+    collect(): T[]
+
+    onEach(action: (it: T) => void): void
+    forEach(action: (it: T) => void): void
+
+    isEmpty(): boolean
+    isNotEmpty(): boolean
+    count(predicate?: (it: T) => boolean): number
+
+    firstIndex(predicate?: (it: T) => boolean): number
+    firstIndexOrNull(predicate?: (it: T) => boolean): number | null
+    lastIndex(predicate?: (it: T) => boolean): number
+    lastIndexOrNull(predicate?: (it: T) => boolean): number | null
+    indices(): Sequence<number>
+    withIndex(): Sequence<[number, T]>
+
+    contains(item: T): boolean
+    containsAll(items: Iterable<T>): boolean
+    any(predicate: (it: T) => boolean): boolean
+    all(predicate: (it: T) => boolean): boolean
+    none(predicate: (it: T) => boolean): boolean
+
+    get(index: number): T
+    getOrNull(index: number): T | null
+    first(predicate?: (it: T) => boolean): T
+    firstOrNull(predicate?: (it: T) => boolean): T | null
+    last(predicate?: (it: T) => boolean): T
+    lastOrNull(predicate?: (it: T) => boolean): T | null
+    single(predicate?: (it: T) => boolean): T
+    singleOrNull(predicate?: (it: T) => boolean): T | null
+
+    min(this: Sequence<number>): number
+    minOrNull(this: Sequence<number>): number | null
+    minWith(compare: (a: T, b: T) => number): T
+    minWithOrNull(compare: (a: T, b: T) => number): T | null
+    minBy(transform: (it: T) => number): T
+    minByOrNull(transform: (it: T) => number): T | null
+    minOf(transform: (it: T) => number): number
+    minOfOrNull(transform: (it: T) => number): number | null
+    max(this: Sequence<number>): number
+    maxOrNull(this: Sequence<number>): number | null
+    maxWith(compare: (a: T, b: T) => number): T
+    maxWithOrNull(compare: (a: T, b: T) => number): T | null
+    maxBy(transform: (it: T) => number): T
+    maxByOrNull(transform: (it: T) => number): T | null
+    maxOf(transform: (it: T) => number): number
+    maxOfOrNull(transform: (it: T) => number): number | null
+
+    reduce(accumulate: (acc: T, it: T) => T): T
+    reduceOrNull(accumulate: (acc: T, it: T) => T): T | null
+    fold<R>(initial: R, accumulate: (acc: R, it: T) => R): R
+    scan<R>(initial: R, accumulate: (acc: R, it: T) => R): Sequence<R>
+    sum(this: Sequence<number>): number
+    sumBy(transform: (it: T) => number): number
+    average(this: Sequence<number>): number
+    averageBy(transform: (it: T) => number): number
+
+    plus(items: Iterable<T>): Sequence<T>
+    minus(items: Iterable<T>): Sequence<T>
+
+    filter<R extends T>(predicate: (it: T) => it is R): Sequence<R>
+    filter(predicate: (it: T) => boolean): Sequence<T>
+    filterIndexed<R extends T>(predicate: (index: number, it: T) => it is R): Sequence<R>
+    filterIndexed(predicate: (index: number, it: T) => boolean): Sequence<T>
+    partition(predicate: (it: T) => boolean): [T[], T[]]
+    drop(count: number): Sequence<T>
+    dropWhile(predicate: (it: T) => boolean): Sequence<T>
+    dropUntil(predicate: (it: T) => boolean): Sequence<T>
+    take(count: number): Sequence<T>
+    takeWhile(predicate: (it: T) => boolean): Sequence<T>
+    takeUntil(predicate: (it: T) => boolean): Sequence<T>
+    slice(start: number, end?: number): Sequence<T>
+    distinct(): Sequence<T>
+    distinctBy<K>(transform: (it: T) => K): Sequence<T>
+
+    map<R>(transform: (it: T) => R): Sequence<R>
+    mapIndexed<R>(transform: (index: number, it: T) => R): Sequence<R>
+    flatten<R>(this: Sequence<Iterable<R>>): Sequence<R>
+    flatMap<R>(transform: (it: T) => Iterable<R>): Sequence<R>
+    flatMapIndexed<R>(transform: (index: number, it: T) => Iterable<R>): Sequence<R>
+    associate<K, V>(transform: (it: T) => [K, V]): Sequence<[K, V]>
+    associateBy<K>(transform: (it: T) => K): Sequence<[K, T]>
+    associateWith<V>(transform: (it: T) => V): Sequence<[T, V]>
+    zip<B>(items: Iterable<B>): Sequence<[T, B]>
+    unzip<A, B>(this: Sequence<[A, B]>): [A[], B[]]
+
+    windowed(size: number, step?: number, partial?: boolean): Sequence<T[]>
+    chunked(size: number): Sequence<T[]>
+
+    groupBy<K>(transform: (it: T) => K): [K, T[]][]
+
+    reversed(): T[]
+    sorted(this: Sequence<number>): number[]
+    sortedWith(compare: (a: T, b: T) => number): T[]
+    sortedBy(transform: (it: T) => number): T[]
+
+    joinToString(separator: string, transform?: (it: T) => string): string
+}
+
+export function sequence<T>(value: Iterable<T>): Sequence<T> {
+    return wrap(() => value);
+}
+
+export function wrap<T>(factory: () => Iterable<T>): Sequence<T> {
+    return {
+        [Symbol.iterator]() {
+            return factory()[Symbol.iterator]();
+        },
+        collect() {
+            return collect(factory());
+        },
+        onEach(action) {
+            return wrap(() => onEach(factory(), action));
+        },
+        forEach(action) {
+            forEach(factory(), action);
+        },
+        isEmpty() {
+            return isEmpty(factory());
+        },
+        isNotEmpty() {
+            return isNotEmpty(factory());
+        },
+        count(predicate) {
+            return count(factory(), predicate);
+        },
+        firstIndex(predicate) {
+            return firstIndex(factory(), predicate);
+        },
+        firstIndexOrNull(predicate) {
+            return firstIndexOrNull(factory(), predicate);
+        },
+        lastIndex(predicate) {
+            return lastIndex(factory(), predicate);
+        },
+        lastIndexOrNull(predicate) {
+            return lastIndexOrNull(factory(), predicate);
+        },
+        indices() {
+            return wrap(() => indices(factory()));
+        },
+        withIndex() {
+            return wrap(() => withIndex(factory()));
+        },
+        contains(item) {
+            return contains(factory(), item);
+        },
+        containsAll(items) {
+            return containsAll(factory(), items);
+        },
+        any(predicate) {
+            return any(factory(), predicate);
+        },
+        all(predicate) {
+            return all(factory(), predicate);
+        },
+        none(predicate) {
+            return none(factory(), predicate);
+        },
+        get(index) {
+            return get(factory(), index);
+        },
+        getOrNull(index) {
+            return getOrNull(factory(), index);
+        },
+        first(predicate) {
+            return first(factory(), predicate);
+        },
+        firstOrNull(predicate) {
+            return firstOrNull(factory(), predicate);
+        },
+        last(predicate) {
+            return last(factory(), predicate);
+        },
+        lastOrNull(predicate) {
+            return lastOrNull(factory(), predicate);
+        },
+        single(predicate) {
+            return single(factory(), predicate);
+        },
+        singleOrNull(predicate) {
+            return singleOrNull(factory(), predicate);
+        },
+        min() {
+            return min(factory() as any);
+        },
+        minOrNull() {
+            return minOrNull(factory() as any);
+        },
+        minWith(compare) {
+            return minWith(factory(), compare);
+        },
+        minWithOrNull(compare) {
+            return minWithOrNull(factory(), compare);
+        },
+        minBy(transform) {
+            return minBy(factory(), transform);
+        },
+        minByOrNull(transform) {
+            return minByOrNull(factory(), transform);
+        },
+        minOf(transform) {
+            return minOf(factory(), transform);
+        },
+        minOfOrNull(transform) {
+            return minOfOrNull(factory(), transform);
+        },
+        max() {
+            return max(factory() as any);
+        },
+        maxOrNull() {
+            return maxOrNull(factory() as any);
+        },
+        maxWith(compare) {
+            return maxWith(factory(), compare);
+        },
+        maxWithOrNull(compare) {
+            return maxWithOrNull(factory(), compare);
+        },
+        maxBy(transform) {
+            return maxBy(factory(), transform);
+        },
+        maxByOrNull(transform) {
+            return maxByOrNull(factory(), transform);
+        },
+        maxOf(transform) {
+            return maxOf(factory(), transform);
+        },
+        maxOfOrNull(transform) {
+            return maxOfOrNull(factory(), transform);
+        },
+        reduce(accumulate) {
+            return reduce(factory(), accumulate);
+        },
+        reduceOrNull(accumulate) {
+            return reduceOrNull(factory(), accumulate);
+        },
+        fold(initial, accumulate) {
+            return fold(factory(), initial, accumulate);
+        },
+        scan(initial, accumulate) {
+            return wrap(() => scan(factory(), initial, accumulate));
+        },
+        sum() {
+            return sum(factory() as any);
+        },
+        sumBy(transform) {
+            return sumBy(factory(), transform);
+        },
+        average() {
+            return average(factory() as any);
+        },
+        averageBy(transform) {
+            return averageBy(factory(), transform);
+        },
+        plus(items) {
+            return wrap(() => plus(factory(), items));
+        },
+        minus(items) {
+            return wrap(() => minus(factory(), items));
+        },
+        filter(predicate: (it: T) => boolean) {
+            return wrap(() => filter(factory(), predicate));
+        },
+        filterIndexed(predicate: (index: number, it: T) => boolean) {
+            return wrap(() => filterIndexed(factory(), predicate));
+        },
+        partition(predicate) {
+            return partition(factory(), predicate);
+        },
+        drop(count) {
+            return wrap(() => drop(factory(), count));
+        },
+        dropWhile(predicate) {
+            return wrap(() => dropWhile(factory(), predicate));
+        },
+        dropUntil(predicate) {
+            return wrap(() => dropUntil(factory(), predicate));
+        },
+        take(count) {
+            return wrap(() => take(factory(), count));
+        },
+        takeWhile(predicate) {
+            return wrap(() => takeWhile(factory(), predicate));
+        },
+        takeUntil(predicate) {
+            return wrap(() => takeUntil(factory(), predicate));
+        },
+        slice(start, end) {
+            return wrap(() => slice(factory(), start, end));
+        },
+        distinct() {
+            return wrap(() => distinct(factory()));
+        },
+        distinctBy(transform) {
+            return wrap(() => distinctBy(factory(), transform));
+        },
+        map(transform) {
+            return wrap(() => map(factory(), transform));
+        },
+        mapIndexed(transform) {
+            return wrap(() => mapIndexed(factory(), transform));
+        },
+        flatten() {
+            return wrap(() => flatten(factory() as any));
+        },
+        flatMap(transform) {
+            return wrap(() => flatMap(factory(), transform));
+        },
+        flatMapIndexed(transform) {
+            return wrap(() => flatMapIndexed(factory(), transform));
+        },
+        associate(transform) {
+            return wrap(() => associate(factory(), transform));
+        },
+        associateBy(transform) {
+            return wrap(() => associateBy(factory(), transform));
+        },
+        associateWith(transform) {
+            return wrap(() => associateWith(factory(), transform));
+        },
+        zip(items) {
+            return wrap(() => zip(factory(), items));
+        },
+        unzip() {
+            return unzip(factory() as any);
+        },
+        windowed(size, step, partial) {
+            return wrap(() => windowed(factory(), size, step, partial));
+        },
+        chunked(size) {
+            return wrap(() => chunked(factory(), size));
+        },
+        groupBy(transform) {
+            return groupBy(factory(), transform);
+        },
+        reversed() {
+            return reversed(factory());
+        },
+        sorted() {
+            return sorted(factory() as any);
+        },
+        sortedWith(compare) {
+            return sortedWith(factory(), compare);
+        },
+        sortedBy(transform) {
+            return sortedBy(factory(), transform);
+        },
+        joinToString(separator, transform) {
+            return joinToString(factory(), separator, transform);
+        },
+    };
+}
diff --git a/src/sequence/all.ts b/src/sequence/all.ts
new file mode 100644
index 0000000..222b99e
--- /dev/null
+++ b/src/sequence/all.ts
@@ -0,0 +1,6 @@
+export function all<T>(value: Iterable<T>, predicate: (it: T) => boolean): boolean {
+    for (const it of value) {
+        if (!predicate(it)) return false;
+    }
+    return true;
+}
diff --git a/src/sequence/any.ts b/src/sequence/any.ts
new file mode 100644
index 0000000..f24eb72
--- /dev/null
+++ b/src/sequence/any.ts
@@ -0,0 +1,6 @@
+export function any<T>(value: Iterable<T>, predicate: (it: T) => boolean): boolean {
+    for (const it of value) {
+        if (predicate(it)) return true;
+    }
+    return false;
+}
diff --git a/src/sequence/associate.ts b/src/sequence/associate.ts
new file mode 100644
index 0000000..2458ee4
--- /dev/null
+++ b/src/sequence/associate.ts
@@ -0,0 +1,5 @@
+export function* associate<T, K, V>(value: Iterable<T>, transform: (it: T) => [K, V]): Iterable<[K, V]> {
+    for (const it of value) {
+        yield transform(it);
+    }
+}
diff --git a/src/sequence/associateBy.ts b/src/sequence/associateBy.ts
new file mode 100644
index 0000000..93cd1a7
--- /dev/null
+++ b/src/sequence/associateBy.ts
@@ -0,0 +1,5 @@
+export function* associateBy<T, K>(value: Iterable<T>, transform: (it: T) => K): Iterable<[K, T]> {
+    for (const it of value) {
+        yield [transform(it), it];
+    }
+}
diff --git a/src/sequence/associateWith.ts b/src/sequence/associateWith.ts
new file mode 100644
index 0000000..5579acc
--- /dev/null
+++ b/src/sequence/associateWith.ts
@@ -0,0 +1,5 @@
+export function* associateWith<T, V>(value: Iterable<T>, transform: (it: T) => V): Iterable<[T, V]> {
+    for (const it of value) {
+        yield [it, transform(it)];
+    }
+}
diff --git a/src/sequence/average.ts b/src/sequence/average.ts
new file mode 100644
index 0000000..c35528f
--- /dev/null
+++ b/src/sequence/average.ts
@@ -0,0 +1,9 @@
+export function average(value: Iterable<number>): number {
+    let count = 0;
+    let sum = 0;
+    for (const it of value) {
+        count++;
+        sum += it;
+    }
+    return count !== 0 ? sum / count : 0;
+}
diff --git a/src/sequence/averageBy.ts b/src/sequence/averageBy.ts
new file mode 100644
index 0000000..5325254
--- /dev/null
+++ b/src/sequence/averageBy.ts
@@ -0,0 +1,9 @@
+export function averageBy<T>(value: Iterable<T>, transform: (it: T) => number): number {
+    let count = 0;
+    let sum = 0;
+    for (const it of value) {
+        count++;
+        sum += transform(it);
+    }
+    return count !== 0 ? sum / count : 0;
+}
diff --git a/src/sequence/chunked.ts b/src/sequence/chunked.ts
new file mode 100644
index 0000000..f22fd9f
--- /dev/null
+++ b/src/sequence/chunked.ts
@@ -0,0 +1,11 @@
+export function* chunked<T>(value: Iterable<T>, size: number): Iterable<T[]> {
+    let buffer: T[] = [];
+    for (const it of value) {
+        buffer.push(it);
+        if (buffer.length === size) {
+            yield buffer;
+            buffer = [];
+        }
+    }
+    if (buffer.length !== 0) yield buffer;
+}
diff --git a/src/sequence/collect.ts b/src/sequence/collect.ts
new file mode 100644
index 0000000..bc52b36
--- /dev/null
+++ b/src/sequence/collect.ts
@@ -0,0 +1,3 @@
+export function collect<T>(value: Iterable<T>): T[] {
+    return [...value];
+}
diff --git a/src/sequence/contains.ts b/src/sequence/contains.ts
new file mode 100644
index 0000000..8c2a39b
--- /dev/null
+++ b/src/sequence/contains.ts
@@ -0,0 +1,6 @@
+export function contains<T>(value: Iterable<T>, item: T): boolean {
+    for (const it of value) {
+        if (it === item) return true;
+    }
+    return false;
+}
diff --git a/src/sequence/containsAll.ts b/src/sequence/containsAll.ts
new file mode 100644
index 0000000..9ae4595
--- /dev/null
+++ b/src/sequence/containsAll.ts
@@ -0,0 +1,7 @@
+export function containsAll<T>(value: Iterable<T>, items: Iterable<T>): boolean {
+    const set = new Set(items);
+    for (const it of value) {
+        set.delete(it);
+    }
+    return set.size === 0;
+}
diff --git a/src/sequence/count.ts b/src/sequence/count.ts
new file mode 100644
index 0000000..98ecd63
--- /dev/null
+++ b/src/sequence/count.ts
@@ -0,0 +1,7 @@
+export function count<T>(value: Iterable<T>, predicate?: (it: T) => boolean): number {
+    let count = 0;
+    for (const it of value) {
+        if (!predicate || predicate(it)) count++;
+    }
+    return count;
+}
diff --git a/src/sequence/distinct.ts b/src/sequence/distinct.ts
new file mode 100644
index 0000000..d4b9087
--- /dev/null
+++ b/src/sequence/distinct.ts
@@ -0,0 +1,8 @@
+export function* distinct<T>(value: Iterable<T>): Iterable<T> {
+    const set: Set<T> = new Set();
+    for (const it of value) {
+        if (set.has(it)) continue;
+        set.add(it);
+        yield it;
+    }
+}
diff --git a/src/sequence/distinctBy.ts b/src/sequence/distinctBy.ts
new file mode 100644
index 0000000..7f078e4
--- /dev/null
+++ b/src/sequence/distinctBy.ts
@@ -0,0 +1,9 @@
+export function* distinctBy<T, K>(value: Iterable<T>, transform: (it: T) => K): Iterable<T> {
+    const set: Set<K> = new Set();
+    for (const it of value) {
+        const x = transform(it);
+        if (set.has(x)) continue;
+        set.add(x);
+        yield it;
+    }
+}
diff --git a/src/sequence/drop.ts b/src/sequence/drop.ts
new file mode 100644
index 0000000..c8a1567
--- /dev/null
+++ b/src/sequence/drop.ts
@@ -0,0 +1,7 @@
+export function* drop<T>(value: Iterable<T>, count: number): Iterable<T> {
+    let dropped = 0;
+    for (const it of value) {
+        if (dropped === count) yield it;
+        else dropped++;
+    }
+}
diff --git a/src/sequence/dropUntil.ts b/src/sequence/dropUntil.ts
new file mode 100644
index 0000000..305329c
--- /dev/null
+++ b/src/sequence/dropUntil.ts
@@ -0,0 +1,12 @@
+export function* dropUntil<T>(value: Iterable<T>, predicate: (it: T) => boolean): Iterable<T> {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    while (!done) {
+        if (predicate(it)) break;
+        ({value: it, done} = iterator.next());
+    }
+    while (!done) {
+        yield it;
+        ({value: it, done} = iterator.next());
+    }
+}
diff --git a/src/sequence/dropWhile.ts b/src/sequence/dropWhile.ts
new file mode 100644
index 0000000..94af9d2
--- /dev/null
+++ b/src/sequence/dropWhile.ts
@@ -0,0 +1,12 @@
+export function* dropWhile<T>(value: Iterable<T>, predicate: (it: T) => boolean): Iterable<T> {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    while (!done) {
+        if (!predicate(it)) break;
+        ({value: it, done} = iterator.next());
+    }
+    while (!done) {
+        yield it;
+        ({value: it, done} = iterator.next());
+    }
+}
diff --git a/src/sequence/filter.ts b/src/sequence/filter.ts
new file mode 100644
index 0000000..e151167
--- /dev/null
+++ b/src/sequence/filter.ts
@@ -0,0 +1,5 @@
+export function* filter<T>(value: Iterable<T>, predicate: (it: T) => boolean): Iterable<T> {
+    for (const it of value) {
+        if (predicate(it)) yield it;
+    }
+}
diff --git a/src/sequence/filterIndexed.ts b/src/sequence/filterIndexed.ts
new file mode 100644
index 0000000..a5850b0
--- /dev/null
+++ b/src/sequence/filterIndexed.ts
@@ -0,0 +1,6 @@
+export function* filterIndexed<T>(value: Iterable<T>, predicate: (index: number, it: T) => boolean): Iterable<T> {
+    let index = 0;
+    for (const it of value) {
+        if (predicate(index++, it)) yield it;
+    }
+}
diff --git a/src/sequence/first.ts b/src/sequence/first.ts
new file mode 100644
index 0000000..dda1b8d
--- /dev/null
+++ b/src/sequence/first.ts
@@ -0,0 +1,8 @@
+import {error} from "../util/error.js";
+
+export function first<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T {
+    for (const it of value) {
+        if (!predicate || predicate(it)) return it;
+    }
+    error();
+}
diff --git a/src/sequence/firstIndex.ts b/src/sequence/firstIndex.ts
new file mode 100644
index 0000000..32f1d9b
--- /dev/null
+++ b/src/sequence/firstIndex.ts
@@ -0,0 +1,10 @@
+import {error} from "../util/error.js";
+
+export function firstIndex<T>(value: Iterable<T>, predicate?: (it: T) => boolean): number {
+    let index = 0;
+    for (const it of value) {
+        if (!predicate || predicate(it)) return index;
+        index++;
+    }
+    error();
+}
diff --git a/src/sequence/firstIndexOrNull.ts b/src/sequence/firstIndexOrNull.ts
new file mode 100644
index 0000000..9e5b883
--- /dev/null
+++ b/src/sequence/firstIndexOrNull.ts
@@ -0,0 +1,8 @@
+export function firstIndexOrNull<T>(value: Iterable<T>, predicate?: (it: T) => boolean): number | null {
+    let index = 0;
+    for (const it of value) {
+        if (!predicate || predicate(it)) return index;
+        index++;
+    }
+    return null;
+}
diff --git a/src/sequence/firstOrNull.ts b/src/sequence/firstOrNull.ts
new file mode 100644
index 0000000..d47476e
--- /dev/null
+++ b/src/sequence/firstOrNull.ts
@@ -0,0 +1,6 @@
+export function firstOrNull<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T | null {
+    for (const it of value) {
+        if (!predicate || predicate(it)) return it;
+    }
+    return null;
+}
diff --git a/src/sequence/flatMap.ts b/src/sequence/flatMap.ts
new file mode 100644
index 0000000..bc5d12b
--- /dev/null
+++ b/src/sequence/flatMap.ts
@@ -0,0 +1,5 @@
+export function* flatMap<T, R>(value: Iterable<T>, transform: (it: T) => Iterable<R>): Iterable<R> {
+    for (const it of value) {
+        yield* transform(it);
+    }
+}
diff --git a/src/sequence/flatMapIndexed.ts b/src/sequence/flatMapIndexed.ts
new file mode 100644
index 0000000..cc573e7
--- /dev/null
+++ b/src/sequence/flatMapIndexed.ts
@@ -0,0 +1,6 @@
+export function* flatMapIndexed<T, R>(value: Iterable<T>, transform: (index: number, it: T) => Iterable<R>): Iterable<R> {
+    let index = 0;
+    for (const it of value) {
+        yield* transform(index++, it);
+    }
+}
diff --git a/src/sequence/flatten.ts b/src/sequence/flatten.ts
new file mode 100644
index 0000000..83147f2
--- /dev/null
+++ b/src/sequence/flatten.ts
@@ -0,0 +1,5 @@
+export function* flatten<R>(value: Iterable<Iterable<R>>): Iterable<R> {
+    for (const it of value) {
+        yield* it;
+    }
+}
diff --git a/src/sequence/fold.ts b/src/sequence/fold.ts
new file mode 100644
index 0000000..cf91f53
--- /dev/null
+++ b/src/sequence/fold.ts
@@ -0,0 +1,7 @@
+export function fold<T, R>(value: Iterable<T>, initial: R, accumulate: (acc: R, it: T) => R): R {
+    let acc = initial;
+    for (const it of value) {
+        acc = accumulate(acc, it);
+    }
+    return acc;
+}
diff --git a/src/sequence/forEach.ts b/src/sequence/forEach.ts
new file mode 100644
index 0000000..70716f9
--- /dev/null
+++ b/src/sequence/forEach.ts
@@ -0,0 +1,5 @@
+export function forEach<T>(value: Iterable<T>, action: (it: T) => void) {
+    for (const it of value) {
+        action(it);
+    }
+}
diff --git a/src/sequence/get.ts b/src/sequence/get.ts
new file mode 100644
index 0000000..682ac3e
--- /dev/null
+++ b/src/sequence/get.ts
@@ -0,0 +1,9 @@
+import {error} from "../util/error.js";
+
+export function get<T>(value: Iterable<T>, index: number): T {
+    let i = 0;
+    for (const it of value) {
+        if (i++ === index) return it;
+    }
+    error();
+}
diff --git a/src/sequence/getOrNull.ts b/src/sequence/getOrNull.ts
new file mode 100644
index 0000000..a2a017d
--- /dev/null
+++ b/src/sequence/getOrNull.ts
@@ -0,0 +1,7 @@
+export function getOrNull<T>(value: Iterable<T>, index: number): T | null {
+    let i = 0;
+    for (const it of value) {
+        if (i++ === index) return it;
+    }
+    return null;
+}
diff --git a/src/sequence/groupBy.ts b/src/sequence/groupBy.ts
new file mode 100644
index 0000000..b32e855
--- /dev/null
+++ b/src/sequence/groupBy.ts
@@ -0,0 +1,9 @@
+export function groupBy<T, K>(value: Iterable<T>, transform: (it: T) => K): [K, T[]][] {
+    const map: Map<K, T[]> = new Map();
+    for (const it of value) {
+        const key = transform(it);
+        if (!map.has(key)) map.set(key, []);
+        map.get(key)!.push(it);
+    }
+    return [...map];
+}
diff --git a/src/sequence/indices.ts b/src/sequence/indices.ts
new file mode 100644
index 0000000..da86ccb
--- /dev/null
+++ b/src/sequence/indices.ts
@@ -0,0 +1,6 @@
+export function* indices(value: Iterable<unknown>): Iterable<number> {
+    let index = 0;
+    for (const _ of value) {
+        yield index++;
+    }
+}
diff --git a/src/sequence/isEmpty.ts b/src/sequence/isEmpty.ts
new file mode 100644
index 0000000..8e77600
--- /dev/null
+++ b/src/sequence/isEmpty.ts
@@ -0,0 +1,4 @@
+export function isEmpty<T>(value: Iterable<T>): boolean {
+    const {done} = value[Symbol.iterator]().next();
+    return done ?? false;
+}
diff --git a/src/sequence/isNotEmpty.ts b/src/sequence/isNotEmpty.ts
new file mode 100644
index 0000000..a7a1b90
--- /dev/null
+++ b/src/sequence/isNotEmpty.ts
@@ -0,0 +1,4 @@
+export function isNotEmpty<T>(value: Iterable<T>): boolean {
+    const {done} = value[Symbol.iterator]().next();
+    return !(done ?? false);
+}
diff --git a/src/sequence/joinToString.ts b/src/sequence/joinToString.ts
new file mode 100644
index 0000000..64b381e
--- /dev/null
+++ b/src/sequence/joinToString.ts
@@ -0,0 +1,7 @@
+export function joinToString<T>(value: Iterable<T>, separator: string, transform?: (it: T) => string): string {
+    const transformed: string[] = [];
+    for (const it of value) {
+        transformed.push(transform ? transform(it) : String(it));
+    }
+    return transformed.join(separator);
+}
diff --git a/src/sequence/last.ts b/src/sequence/last.ts
new file mode 100644
index 0000000..2077529
--- /dev/null
+++ b/src/sequence/last.ts
@@ -0,0 +1,13 @@
+import {error} from "../util/error.js";
+
+export function last<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T {
+    let last!: T;
+    let found = false;
+    for (const it of value) {
+        if (predicate && !predicate(it)) continue;
+        last = it;
+        found = true;
+    }
+    if (!found) error();
+    return last;
+}
diff --git a/src/sequence/lastIndex.ts b/src/sequence/lastIndex.ts
new file mode 100644
index 0000000..a6c1dba
--- /dev/null
+++ b/src/sequence/lastIndex.ts
@@ -0,0 +1,12 @@
+import {error} from "../util/error.js";
+
+export function lastIndex<T>(value: Iterable<T>, predicate?: (it: T) => boolean): number {
+    let last: number | undefined;
+    let index = 0;
+    for (const it of value) {
+        if (!predicate || predicate(it)) last = index;
+        index++;
+    }
+    if (last === undefined) error();
+    return last;
+}
diff --git a/src/sequence/lastIndexOrNull.ts b/src/sequence/lastIndexOrNull.ts
new file mode 100644
index 0000000..e9a44e6
--- /dev/null
+++ b/src/sequence/lastIndexOrNull.ts
@@ -0,0 +1,10 @@
+export function lastIndexOrNull<T>(value: Iterable<T>, predicate?: (it: T) => boolean): number | null {
+    let last: number | undefined;
+    let index = 0;
+    for (const it of value) {
+        if (!predicate || predicate(it)) last = index;
+        index++;
+    }
+    if (last === undefined) return null;
+    return last;
+}
diff --git a/src/sequence/lastOrNull.ts b/src/sequence/lastOrNull.ts
new file mode 100644
index 0000000..012bcb5
--- /dev/null
+++ b/src/sequence/lastOrNull.ts
@@ -0,0 +1,11 @@
+export function lastOrNull<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T | null {
+    let last!: T;
+    let found = false;
+    for (const it of value) {
+        if (predicate && !predicate(it)) continue;
+        last = it;
+        found = true;
+    }
+    if (!found) return null;
+    return last;
+}
diff --git a/src/sequence/map.ts b/src/sequence/map.ts
new file mode 100644
index 0000000..02b5348
--- /dev/null
+++ b/src/sequence/map.ts
@@ -0,0 +1,5 @@
+export function* map<T, R>(value: Iterable<T>, transform: (it: T) => R): Iterable<R> {
+    for (const it of value) {
+        yield transform(it);
+    }
+}
diff --git a/src/sequence/mapIndexed.ts b/src/sequence/mapIndexed.ts
new file mode 100644
index 0000000..d9870f6
--- /dev/null
+++ b/src/sequence/mapIndexed.ts
@@ -0,0 +1,6 @@
+export function* mapIndexed<T, R>(value: Iterable<T>, transform: (index: number, it: T) => R): Iterable<R> {
+    let index = 0;
+    for (const it of value) {
+        yield transform(index++, it);
+    }
+}
diff --git a/src/sequence/max.ts b/src/sequence/max.ts
new file mode 100644
index 0000000..103865b
--- /dev/null
+++ b/src/sequence/max.ts
@@ -0,0 +1,13 @@
+import {error} from "../util/error.js";
+
+export function max(value: Iterable<number>): number {
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        if (found && it <= max) continue;
+        max = it;
+        found = true;
+    }
+    if (!found) error();
+    return max;
+}
diff --git a/src/sequence/maxBy.ts b/src/sequence/maxBy.ts
new file mode 100644
index 0000000..e1ffa9b
--- /dev/null
+++ b/src/sequence/maxBy.ts
@@ -0,0 +1,16 @@
+import {error} from "../util/error.js";
+
+export function maxBy<T>(value: Iterable<T>, transform: (it: T) => number): T {
+    let maxIt!: T;
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val <= max) continue;
+        maxIt = it;
+        max = val;
+        found = true;
+    }
+    if (!found) error();
+    return maxIt;
+}
diff --git a/src/sequence/maxByOrNull.ts b/src/sequence/maxByOrNull.ts
new file mode 100644
index 0000000..33aa045
--- /dev/null
+++ b/src/sequence/maxByOrNull.ts
@@ -0,0 +1,14 @@
+export function maxByOrNull<T>(value: Iterable<T>, transform: (it: T) => number): T | null {
+    let maxIt!: T;
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val <= max) continue;
+        maxIt = it;
+        max = val;
+        found = true;
+    }
+    if (!found) return null;
+    return maxIt;
+}
diff --git a/src/sequence/maxOf.ts b/src/sequence/maxOf.ts
new file mode 100644
index 0000000..c7dd585
--- /dev/null
+++ b/src/sequence/maxOf.ts
@@ -0,0 +1,14 @@
+import {error} from "../util/error.js";
+
+export function maxOf<T>(value: Iterable<T>, transform: (it: T) => number): number {
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val <= max) continue;
+        max = val;
+        found = true;
+    }
+    if (!found) error();
+    return max;
+}
diff --git a/src/sequence/maxOfOrNull.ts b/src/sequence/maxOfOrNull.ts
new file mode 100644
index 0000000..49f165a
--- /dev/null
+++ b/src/sequence/maxOfOrNull.ts
@@ -0,0 +1,12 @@
+export function maxOfOrNull<T>(value: Iterable<T>, transform: (it: T) => number): number | null {
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val <= max) continue;
+        max = val;
+        found = true;
+    }
+    if (!found) return null;
+    return max;
+}
diff --git a/src/sequence/maxOrNull.ts b/src/sequence/maxOrNull.ts
new file mode 100644
index 0000000..fc8b767
--- /dev/null
+++ b/src/sequence/maxOrNull.ts
@@ -0,0 +1,11 @@
+export function maxOrNull(value: Iterable<number>): number | null {
+    let max!: number;
+    let found = false;
+    for (const it of value) {
+        if (found && it <= max) continue;
+        max = it;
+        found = true;
+    }
+    if (!found) return null;
+    return max;
+}
diff --git a/src/sequence/maxWith.ts b/src/sequence/maxWith.ts
new file mode 100644
index 0000000..af9cda8
--- /dev/null
+++ b/src/sequence/maxWith.ts
@@ -0,0 +1,13 @@
+import {error} from "../util/error.js";
+
+export function maxWith<T>(value: Iterable<T>, compare: (a: T, b: T) => number): T {
+    let max!: T;
+    let found = false;
+    for (const it of value) {
+        if (found && compare(it, max) <= 0) continue;
+        max = it;
+        found = true;
+    }
+    if (!found) error();
+    return max;
+}
diff --git a/src/sequence/maxWithOrNull.ts b/src/sequence/maxWithOrNull.ts
new file mode 100644
index 0000000..6182181
--- /dev/null
+++ b/src/sequence/maxWithOrNull.ts
@@ -0,0 +1,11 @@
+export function maxWithOrNull<T>(value: Iterable<T>, compare: (a: T, b: T) => number): T | null {
+    let max!: T;
+    let found = false;
+    for (const it of value) {
+        if (found && compare(it, max) <= 0) continue;
+        max = it;
+        found = true;
+    }
+    if (!found) return null;
+    return max;
+}
diff --git a/src/sequence/min.ts b/src/sequence/min.ts
new file mode 100644
index 0000000..eebc0fc
--- /dev/null
+++ b/src/sequence/min.ts
@@ -0,0 +1,13 @@
+import {error} from "../util/error.js";
+
+export function min(value: Iterable<number>): number {
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        if (found && it >= min) continue;
+        min = it;
+        found = true;
+    }
+    if (!found) error();
+    return min;
+}
diff --git a/src/sequence/minBy.ts b/src/sequence/minBy.ts
new file mode 100644
index 0000000..5b0be99
--- /dev/null
+++ b/src/sequence/minBy.ts
@@ -0,0 +1,16 @@
+import {error} from "../util/error.js";
+
+export function minBy<T>(value: Iterable<T>, transform: (it: T) => number): T {
+    let minIt!: T;
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val >= min) continue;
+        minIt = it;
+        min = val;
+        found = true;
+    }
+    if (!found) error();
+    return minIt;
+}
diff --git a/src/sequence/minByOrNull.ts b/src/sequence/minByOrNull.ts
new file mode 100644
index 0000000..e220586
--- /dev/null
+++ b/src/sequence/minByOrNull.ts
@@ -0,0 +1,14 @@
+export function minByOrNull<T>(value: Iterable<T>, transform: (it: T) => number): T | null {
+    let minIt!: T;
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val >= min) continue;
+        minIt = it;
+        min = val;
+        found = true;
+    }
+    if (!found) return null;
+    return minIt;
+}
diff --git a/src/sequence/minOf.ts b/src/sequence/minOf.ts
new file mode 100644
index 0000000..24bd182
--- /dev/null
+++ b/src/sequence/minOf.ts
@@ -0,0 +1,14 @@
+import {error} from "../util/error.js";
+
+export function minOf<T>(value: Iterable<T>, transform: (it: T) => number): number {
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val >= min) continue;
+        min = val;
+        found = true;
+    }
+    if (!found) error();
+    return min;
+}
diff --git a/src/sequence/minOfOrNull.ts b/src/sequence/minOfOrNull.ts
new file mode 100644
index 0000000..afbd75a
--- /dev/null
+++ b/src/sequence/minOfOrNull.ts
@@ -0,0 +1,12 @@
+export function minOfOrNull<T>(value: Iterable<T>, transform: (it: T) => number): number | null {
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        const val = transform(it);
+        if (found && val >= min) continue;
+        min = val;
+        found = true;
+    }
+    if (!found) return null;
+    return min;
+}
diff --git a/src/sequence/minOrNull.ts b/src/sequence/minOrNull.ts
new file mode 100644
index 0000000..906e955
--- /dev/null
+++ b/src/sequence/minOrNull.ts
@@ -0,0 +1,11 @@
+export function minOrNull(value: Iterable<number>): number | null {
+    let min!: number;
+    let found = false;
+    for (const it of value) {
+        if (found && it <= min) continue;
+        min = it;
+        found = true;
+    }
+    if (!found) return null;
+    return min;
+}
diff --git a/src/sequence/minWith.ts b/src/sequence/minWith.ts
new file mode 100644
index 0000000..d2904fd
--- /dev/null
+++ b/src/sequence/minWith.ts
@@ -0,0 +1,14 @@
+import {error} from "../util/error.js";
+
+export function minWith<T>(value: Iterable<T>, compare: (a: T, b: T) => number): T {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    if (done) error();
+    let min = it;
+    ({value: it, done} = iterator.next());
+    while (!done) {
+        if (compare(it, min) < 0) min = it;
+        ({value: it, done} = iterator.next());
+    }
+    return min;
+}
diff --git a/src/sequence/minWithOrNull.ts b/src/sequence/minWithOrNull.ts
new file mode 100644
index 0000000..5e11acc
--- /dev/null
+++ b/src/sequence/minWithOrNull.ts
@@ -0,0 +1,12 @@
+export function minWithOrNull<T>(value: Iterable<T>, compare: (a: T, b: T) => number): T | null {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    if (done) return null;
+    let min = it;
+    ({value: it, done} = iterator.next());
+    while (!done) {
+        if (compare(it, min) < 0) min = it;
+        ({value: it, done} = iterator.next());
+    }
+    return min;
+}
diff --git a/src/sequence/minus.ts b/src/sequence/minus.ts
new file mode 100644
index 0000000..c2f698f
--- /dev/null
+++ b/src/sequence/minus.ts
@@ -0,0 +1,6 @@
+export function* minus<T>(value: Iterable<T>, items: Iterable<T>): Iterable<T> {
+    const set = new Set(items);
+    for (const it of value) {
+        if (!set.has(it)) yield it;
+    }
+}
diff --git a/src/sequence/none.ts b/src/sequence/none.ts
new file mode 100644
index 0000000..452e929
--- /dev/null
+++ b/src/sequence/none.ts
@@ -0,0 +1,6 @@
+export function none<T>(value: Iterable<T>, predicate: (it: T) => boolean): boolean {
+    for (const it of value) {
+        if (predicate(it)) return false;
+    }
+    return true;
+}
diff --git a/src/sequence/onEach.ts b/src/sequence/onEach.ts
new file mode 100644
index 0000000..582ed6a
--- /dev/null
+++ b/src/sequence/onEach.ts
@@ -0,0 +1,6 @@
+export function* onEach<T>(value: Iterable<T>, action: (it: T) => void): Iterable<T> {
+    for (const it of value) {
+        action(it);
+        yield it;
+    }
+}
diff --git a/src/sequence/partition.ts b/src/sequence/partition.ts
new file mode 100644
index 0000000..7ee920b
--- /dev/null
+++ b/src/sequence/partition.ts
@@ -0,0 +1,9 @@
+export function partition<T>(value: Iterable<T>, predicate: (it: T) => boolean): [T[], T[]] {
+    const t: T[] = [];
+    const f: T[] = [];
+    for (const it of value) {
+        if (predicate(it)) t.push(it);
+        else f.push(it);
+    }
+    return [t, f];
+}
diff --git a/src/sequence/plus.ts b/src/sequence/plus.ts
new file mode 100644
index 0000000..ba320c1
--- /dev/null
+++ b/src/sequence/plus.ts
@@ -0,0 +1,8 @@
+export function* plus<T>(value1: Iterable<T>, value2: Iterable<T>): Iterable<T> {
+    for (const it of value1) {
+        yield it;
+    }
+    for (const it of value2) {
+        yield it;
+    }
+}
diff --git a/src/sequence/reduce.ts b/src/sequence/reduce.ts
new file mode 100644
index 0000000..e19a97d
--- /dev/null
+++ b/src/sequence/reduce.ts
@@ -0,0 +1,14 @@
+import {error} from "../util/error.js";
+
+export function reduce<T>(value: Iterable<T>, accumulate: (acc: T, it: T) => T): T {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    if (done) error();
+    let acc = it
+    ({value: it, done} = iterator.next());
+    while (!done) {
+        acc = accumulate(acc, it);
+        ({value: it, done} = iterator.next());
+    }
+    return acc;
+}
diff --git a/src/sequence/reduceOrNull.ts b/src/sequence/reduceOrNull.ts
new file mode 100644
index 0000000..8455f3f
--- /dev/null
+++ b/src/sequence/reduceOrNull.ts
@@ -0,0 +1,12 @@
+export function reduceOrNull<T>(value: Iterable<T>, accumulate: (acc: T, it: T) => T): T | null {
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    if (done) return null;
+    let acc = it
+    ({value: it, done} = iterator.next());
+    while (!done) {
+        acc = accumulate(acc, it);
+        ({value: it, done} = iterator.next());
+    }
+    return acc;
+}
diff --git a/src/sequence/reversed.ts b/src/sequence/reversed.ts
new file mode 100644
index 0000000..2a1eb88
--- /dev/null
+++ b/src/sequence/reversed.ts
@@ -0,0 +1,3 @@
+export function reversed<T>(iterable: Iterable<T>): T[] {
+    return [...iterable].reverse();
+}
diff --git a/src/sequence/scan.ts b/src/sequence/scan.ts
new file mode 100644
index 0000000..f4f2305
--- /dev/null
+++ b/src/sequence/scan.ts
@@ -0,0 +1,8 @@
+export function* scan<T, R>(value: Iterable<T>, initial: R, accumulate: (acc: R, it: T) => R): Iterable<R> {
+    let acc = initial;
+    yield acc;
+    for (const it of value) {
+        acc = accumulate(acc, it);
+        yield acc;
+    }
+}
diff --git a/src/sequence/single.ts b/src/sequence/single.ts
new file mode 100644
index 0000000..9db0d4f
--- /dev/null
+++ b/src/sequence/single.ts
@@ -0,0 +1,14 @@
+import {error} from "../util/error.js";
+
+export function single<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T {
+    let last!: T;
+    let found = false;
+    for (const it of value) {
+        if (predicate && !predicate(it)) continue;
+        if (found) error();
+        last = it;
+        found = true;
+    }
+    if (!found) error();
+    return last;
+}
diff --git a/src/sequence/singleOrNull.ts b/src/sequence/singleOrNull.ts
new file mode 100644
index 0000000..417b830
--- /dev/null
+++ b/src/sequence/singleOrNull.ts
@@ -0,0 +1,12 @@
+export function singleOrNull<T>(value: Iterable<T>, predicate?: (it: T) => boolean): T | null {
+    let last!: T;
+    let found = false;
+    for (const it of value) {
+        if (predicate && !predicate(it)) continue;
+        if (found) return null;
+        last = it;
+        found = true;
+    }
+    if (!found) return null;
+    return last;
+}
diff --git a/src/sequence/slice.ts b/src/sequence/slice.ts
new file mode 100644
index 0000000..9cf7144
--- /dev/null
+++ b/src/sequence/slice.ts
@@ -0,0 +1,7 @@
+export function* slice<T>(value: Iterable<T>, start: number, endInclusive?: number): Iterable<T> {
+    let index = 0;
+    for (const it of value) {
+        if (endInclusive !== undefined && index > endInclusive) return;
+        if (index++ >= start) yield it;
+    }
+}
diff --git a/src/sequence/sorted.ts b/src/sequence/sorted.ts
new file mode 100644
index 0000000..ea0909e
--- /dev/null
+++ b/src/sequence/sorted.ts
@@ -0,0 +1,3 @@
+export function sorted(iterable: Iterable<number>): number[] {
+    return [...iterable].sort((a, b) => a - b);
+}
diff --git a/src/sequence/sortedBy.ts b/src/sequence/sortedBy.ts
new file mode 100644
index 0000000..adfa7fc
--- /dev/null
+++ b/src/sequence/sortedBy.ts
@@ -0,0 +1,3 @@
+export function sortedBy<T>(iterable: Iterable<T>, transform: (it: T) => number): T[] {
+    return [...iterable].sort((a, b) => transform(a) - transform(b));
+}
diff --git a/src/sequence/sortedWith.ts b/src/sequence/sortedWith.ts
new file mode 100644
index 0000000..5ffff0b
--- /dev/null
+++ b/src/sequence/sortedWith.ts
@@ -0,0 +1,3 @@
+export function sortedWith<T>(iterable: Iterable<T>, compare: (a: T, b: T) => number): T[] {
+    return [...iterable].sort(compare);
+}
diff --git a/src/sequence/sum.ts b/src/sequence/sum.ts
new file mode 100644
index 0000000..d4baa12
--- /dev/null
+++ b/src/sequence/sum.ts
@@ -0,0 +1,7 @@
+export function sum(value: Iterable<number>): number {
+    let sum = 0;
+    for (const it of value) {
+        sum += it;
+    }
+    return sum;
+}
diff --git a/src/sequence/sumBy.ts b/src/sequence/sumBy.ts
new file mode 100644
index 0000000..feabcf2
--- /dev/null
+++ b/src/sequence/sumBy.ts
@@ -0,0 +1,7 @@
+export function sumBy<T>(value: Iterable<T>, transform: (it: T) => number): number {
+    let sum = 0;
+    for (const it of value) {
+        sum += transform(it);
+    }
+    return sum;
+}
diff --git a/src/sequence/take.ts b/src/sequence/take.ts
new file mode 100644
index 0000000..0ab257f
--- /dev/null
+++ b/src/sequence/take.ts
@@ -0,0 +1,7 @@
+export function* take<T>(value: Iterable<T>, count: number): Iterable<T> {
+    let taken = 0;
+    for (const it of value) {
+        if (taken++ === count) return;
+        yield it;
+    }
+}
diff --git a/src/sequence/takeUntil.ts b/src/sequence/takeUntil.ts
new file mode 100644
index 0000000..2fbcf89
--- /dev/null
+++ b/src/sequence/takeUntil.ts
@@ -0,0 +1,6 @@
+export function* takeUntil<T>(value: Iterable<T>, predicate: (it: T) => boolean): Iterable<T> {
+    for (const it of value) {
+        if (predicate(it)) return;
+        yield it;
+    }
+}
diff --git a/src/sequence/takeWhile.ts b/src/sequence/takeWhile.ts
new file mode 100644
index 0000000..9f5c7f2
--- /dev/null
+++ b/src/sequence/takeWhile.ts
@@ -0,0 +1,6 @@
+export function* takeWhile<T>(value: Iterable<T>, predicate: (it: T) => boolean): Iterable<T> {
+    for (const it of value) {
+        if (!predicate(it)) return;
+        yield it;
+    }
+}
diff --git a/src/sequence/unzip.ts b/src/sequence/unzip.ts
new file mode 100644
index 0000000..2b9cb2f
--- /dev/null
+++ b/src/sequence/unzip.ts
@@ -0,0 +1,12 @@
+export function unzip<A, B>(value: Iterable<[A, B]>): [A[], B[]] {
+    const a: A[] = [];
+    const b: B[] = [];
+    const iterator = value[Symbol.iterator]();
+    let {value: it, done} = iterator.next();
+    while (!done) {
+        a.push(it[0]);
+        b.push(it[1]);
+        ({value: it, done} = iterator.next());
+    }
+    return [a, b];
+}
diff --git a/src/sequence/windowed.ts b/src/sequence/windowed.ts
new file mode 100644
index 0000000..a6cf0c7
--- /dev/null
+++ b/src/sequence/windowed.ts
@@ -0,0 +1,15 @@
+export function* windowed<T>(value: Iterable<T>, size: number, step: number = 1, partial: boolean = false): Iterable<T[]> {
+    const buffer: T[] = [];
+    for (const it of value) {
+        buffer.push(it);
+        if (buffer.length === size) {
+            yield [...buffer];
+            buffer.splice(0, step);
+        }
+    }
+    if (!partial) return;
+    while (buffer.length !== 0) {
+        yield [...buffer];
+        buffer.splice(0, step);
+    }
+}
diff --git a/src/sequence/withIndex.ts b/src/sequence/withIndex.ts
new file mode 100644
index 0000000..ae705f9
--- /dev/null
+++ b/src/sequence/withIndex.ts
@@ -0,0 +1,6 @@
+export function* withIndex<T>(value: Iterable<T>): Iterable<[number, T]> {
+    let index = 0;
+    for (const it of value) {
+        yield [index++, it];
+    }
+}
diff --git a/src/sequence/zip.ts b/src/sequence/zip.ts
new file mode 100644
index 0000000..9d80876
--- /dev/null
+++ b/src/sequence/zip.ts
@@ -0,0 +1,9 @@
+export function* zip<T, B>(value: Iterable<T>, items: Iterable<B>): Iterable<[T, B]> {
+    const iteratorA = value[Symbol.iterator]();
+    const iteratorB = items[Symbol.iterator]();
+    let {value: itA, done: doneA} = iteratorA.next();
+    let {value: itB, done: doneB} = iteratorB.next();
+    while (!doneA && !doneB) {
+        yield [itA, itB];
+    }
+}
diff --git a/src/util/error.ts b/src/util/error.ts
new file mode 100644
index 0000000..fc4dbc5
--- /dev/null
+++ b/src/util/error.ts
@@ -0,0 +1,3 @@
+export function error(message?: string): never {
+    throw Error(message);
+}
diff --git a/tsconfig.json b/tsconfig.json
index 8c1d21f..166eeca 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,7 +1,7 @@
 {
   "compilerOptions": {
-    "moduleResolution": "NodeNext",
     "module": "NodeNext",
+    "moduleResolution": "NodeNext",
     "target": "ESNext",
     "rootDir": "./src/",
     "outDir": "./dist/",