Flow flatten type

flatten.js

declare function flatten<
  A,
  B: $ReadOnlyArray<A> | A,
  C: $ReadOnlyArray<B> | B,
  D: $ReadOnlyArray<C> | C,
  E: $ReadOnlyArray<D> | D,
>(xs: $ReadOnlyArray<E>): Array<A>

const listsList  = [{
  items: [[10, 2], 3, [0, 1, [2], [1, [2]], ],2, [2, 3]]
}]
const items = listsList.map(({ items }) => items)
const ok: number[] = flatten(items)
const t = flatten(items).map(x => x + 1)