code: function Series(executor) {
    if (typeof executor !== 'function') {
      throw new TypeError();
    }
  
    if (!(this instanceof Series)) {
      return new Series(executor);
    }
  
    if (executor !== blessed.be) {
      blessed.be(this, executor, true);
    }
  }
docs: Parallelization controls the number of pending promises (as returned by an iterator callback) an operation is waiting on. Per operation no new callbacks are invoked when this number reaches the maximum concurrency.