docs: function Propagator(
      constructor, transforms,
      cancellable, signalCancelled) {
    this._constructor = constructor || promise.Promise;
    this._transforms = transforms;
    this._cancellable = cancellable === true;
    this._signalCancelled = signalCancelled;
  
    this._state = INITIALIZED;
    this._resolved = false;
    this._cancelled = false;
    this._delegatedResolve = null;
    this._delegatedReject = null;
  
    this._result = null;
    this._resultThen = null;
    this._handledRejection = null;
  
    this._promise = null;
  }