function SumObservable(source, fn) {
      this.source = source;
      this._fn = fn;
      __super__.call(this);
    }
Returns the first index at which a given element can be found in the observable sequence, or -1 if it is not present.
@param {Any} searchElement Element to locate in the array.
@param {Number} [fromIndex] The index to start the search.  If not specified, defaults to 0.
@returns {Observable} And observable sequence containing the first index at which a given element can be found in the observable sequence, or -1 if it is not present.