How to use the observable-fns.Observable.from function in observable-fns

function subscribeToOrderbookUncached(horizonURL: string, sellingAsset: string, buyingAsset: string) {
  const buying = parseAssetID(buyingAsset)
  const selling = parseAssetID(sellingAsset)
  const query = createOrderbookQuery(selling, buying)

  if (selling.equals(buying)) {
    return Observable.from([createEmptyOrderbookRecord(buying, buying)])
  }

  const createURL = () => String(new URL(`/order_book?${qs.stringify({ ...query, cursor: "now" })}`, horizonURL))
  const fetchUpdate = () => fetchOrderbookRecord(horizonURL, sellingAsset, buyingAsset)

  let latestKnownSnapshot = ""

  // TODO: Optimize - Make UpdateT = ValueT & { [$snapshot]: string }

  return subscribeToUpdatesAndPoll({
    async applyUpdate(update) {
      latestKnownSnapshot = JSON.stringify(update)
      return update
    },
    fetchUpdate,
    async init() {

constructor(
    spawnWorker: () => Promise,
    optionsOrSize?: number | PoolOptions
  ) {
    const options: PoolOptions = typeof optionsOrSize === "number"
      ? { size: optionsOrSize }
      : optionsOrSize || {}

    const { size = defaultPoolSize } = options

    this.debug = DebugLogger(`threads:pool:${slugify(options.name || String(nextPoolID++))}`)
    this.options = options
    this.workers = spawnWorkers(spawnWorker, size)

    this.eventObservable = multicast(Observable.from(this.eventSubject))

    Promise.all(this.workers.map(worker => worker.init)).then(
      () => this.eventSubject.next({
        type: PoolEventType.initialized,
        size: this.workers.length
      }),
      error => {
        this.debug("Error while initializing pool worker:", error)
        this.eventSubject.error(error)
        this.initErrors.push(error)
      }
    )
  }

  }).flatMap((txs: Horizon.TransactionResponse[]) => Observable.from(txs))
}