StreamClass.reduce<S, A>(source: Source<S>,fn: (accumulator: A,value: Exclude<S, AnyFailure>,index: number) => A | PromiseLike<A>,initial: A): Task<A, Extract<S, AnyFailure>>
StreamClass#reduce as a static: Stream.reduce(source, …) is
Stream.from(source).reduce(…).
await Stream.reduce([1, 2, 3], (sum, n) => sum + n, 0); // 6