message: impoved SetUtils
  use ReadonlySet when possible
  added .add
diff: /**
  - * Copyright (c) 2018 mol* contributors, licensed under MIT, See LICENSE file for more info.
  + * Copyright (c) 2018-2019 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  */
  export namespace SetUtils {
  /** Test if set a contains all elements of set b. */
  - export function isSuperset<T>(setA: Set<T>, setB: Set<T>) {
  + export function isSuperset<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>) {
  for (const elm of Array.from(setB)) {
  if (!setA.has(elm)) return false;
  }
  return true;
  }
  + /** Add all elements from `sets` to `out` */
  + export function add<T>(out: Set<T>, ...sets: ReadonlySet<T>[]): Set<T> {
  + for (let i = 0; i < sets.length; i++) {
  + for (const elem of Array.from(sets[i])) out.add(elem);
  + }
  + return out;
  + }
  +
  /** Create set containing elements of both set a and set b. */
  - export function union<T>(setA: Set<T>, setB: Set<T>): Set<T> {
  + export function union<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>): Set<T> {
  const union = new Set(setA);
  for (const elem of Array.from(setB)) union.add(elem);
  return union;
  }
  - export function unionMany<T>(...sets: Set<T>[]) {
  + export function unionMany<T>(...sets: ReadonlySet<T>[]) {
  if (sets.length === 0) return new Set<T>();
  - if (sets.length === 1) return sets[0];
  + if (sets.length === 1) new Set(sets[0]);
  const union = new Set(sets[0]);
  for (let i = 1; i < sets.length; i++) {
  for (const elem of Array.from(sets[i])) union.add(elem);
  @@ -42,7 +50,7 @@ export namespace SetUtils {
  }
  /** Create set containing elements of set a that are also in set b. */
  - export function intersection<T>(setA: Set<T>, setB: Set<T>): Set<T> {
  + export function intersection<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>): Set<T> {
  const intersection = new Set<T>();
  for (const elem of Array.from(setB)) {
  if (setA.has(elem)) intersection.add(elem);
  @@ -50,7 +58,7 @@ export namespace SetUtils {
  return intersection;
  }
  - export function areIntersecting<T>(setA: Set<T>, setB: Set<T>): boolean {
  + export function areIntersecting<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>): boolean {
  for (const elem of Array.from(setB)) {
  if (setA.has(elem)) return true;
  }
  @@ -58,14 +66,14 @@ export namespace SetUtils {
  }
  /** Create set containing elements of set a that are not in set b. */
  - export function difference<T>(setA: Set<T>, setB: Set<T>): Set<T> {
  + export function difference<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>): Set<T> {
  const difference = new Set(setA);
  for (const elem of Array.from(setB)) difference.delete(elem);
  return difference;
  }
  /** Test if set a and b contain the same elements. */
  - export function areEqual<T>(setA: Set<T>, setB: Set<T>) {
  + export function areEqual<T>(setA: ReadonlySet<T>, setB: ReadonlySet<T>) {
  if (setA.size !== setB.size) return false
  for (const elm of Array.from(setB)) {
  if (!setA.has(elm)) return false;
  