function CountObservable(source, fn) {
      this.source = source;
      this._fn = fn;
      __super__.call(this);
    }
Determines whether an observable sequence includes a specified element with an optional equality comparer.
@param searchElement The value to locate in the source sequence.
@param {Number} [fromIndex] An equality comparer to compare elements.
@returns {Observable} An observable sequence containing a single element determining whether the source sequence includes an element that has the specified value from the given index.