function addNextIteratorValueToBuffers() {
    'use strict';
    const {done, value} = iterator.next();
    if (done) return false;
    for (let i = 0, buffer; buffer = buffers[i++];) {
      buffer.push(value);
    }
    return true;
  }
@return {boolean} True iff something was added to the buffers, false
    otherwise. Used to signal whether there were any more iterators, or if
    the parent iterator should indicate exhaustion.