scan
function scan<V, A = V>(accumulator: (
acc: A | V,
value: V,
index: number,
) => A): OperatorFunction<V, V | A>;function scan<V, A>(accumulator: (
acc: A,
value: V,
index: number,
) => A, seed: A): OperatorFunction<V, A>;function scan<V, A, S>(accumulator: (
acc: A | S,
value: V,
index: number,
) => A, seed: S): OperatorFunction<V, A>;