function SomeObservable(source, fn) {
      this.source = source;
      this._fn = fn;
      __super__.call(this);
    }
Applies an accumulator function over an observable sequence, returning the result of the aggregation as a single element in the result sequence. The specified seed value is used as the initial accumulator value.
For aggregation behavior with incremental intermediate results, see Observable.scan.
@param {Function} accumulator An accumulator function to be invoked on each element.
@param {Any} [seed] The initial accumulator value.
@returns {Observable} An observable sequence containing a single element with the final accumulator value.