message: add assertUnreachable helper
  to type check if, e.g. if/switch statements are exhaustive
  TODO use...
diff: /**
  - * Copyright (c) 2018-2020 mol* contributors, licensed under MIT, See LICENSE file for more info.
  + * Copyright (c) 2018-2021 mol* contributors, licensed under MIT, See LICENSE file for more info.
  *
  * @author Alexander Rose <alexander.rose@weirdbyte.de>
  * @author David Sehnal <david.sehnal@gmail.com>
  @@ -26,3 +26,7 @@ export function ObjectKeys<T extends object>(o: T) {
  return Object.keys(o) as (keyof T)[];
  }
  export interface FiniteArray<T, L extends number = number> extends ReadonlyArray<T> { length: L };
  +
  +export function assertUnreachable(x: never): never {
  + throw new Error('unreachable');
  +}
  \ No newline at end of file
  