I have a question about the source, 'multipelx async iterator' of Deno standard library

What is purpose of iteratorCount in the following code (from https://deno.land/[email protected]/async/mux_async_iterator.ts)?

#iteratorCount = 0
#yields = []
#throws = []
#signal = deferred()

add () {
  ++this.#iteratorCount
  this.#callIteratorNext(iterator)
}

async #callIteratorNext (iterator) {
  try {
    const { value, done } = await iterator.next()
    if (done) {
      --this.#iteratorCount
    } else {
      this.#yields.push({ iterator, value })
    }
  } catch (e) {
    this.#throws.push(e)
  }
  this.#signal.resolve()
}

async *iterate () {
  while (this.#iteratorCount > 0) {
    await this.#signal

    for (let i = 0; i < this.#yields.length; i++) {
      const { iterator, value } = this.#yields[i]
      yield value

      this.#callIteratorNext(iterator)
    }

    if (this.#throws.length) {
      for (const e of this.#throws) {
        throw e
      }
      this.#throws.length = 0
    }

    this.#yields.length = 0
    this.#signal = deferred()
  }
}

As far as I see, there's nothing to do with iteratorCount if there is no error in all of the iterators added to MuxIterator.

Also, the concept of "not clearing the iteratorCount after an errorhandling" and "loop while (this.iteratorCount > 0)" seems very weird to me.

I've concluded that this kind of stuff is for "manual error handling", in order to make Mux Async Iterator iterates furthermore by adding additional iterators.

Despite that view, I can't think of a situation where iteratorCount go back to zero.

The only way we can decrease the count is to callIteratorNext() and make it done. But, we have to add iterator before it, which makes the count increase.

So, I can't get any role of iteratorCount.