function SingleObserver(o, obj, s) {
      this._o = o;
      this._obj = obj;
      this._s = s;
      this._i = 0;
      this._hv = false;
      this._v = null;
      __super__.call(this);
    }
Returns the element at a specified index in a sequence or default value if not found.
@param {Number} index The zero-based index of the element to retrieve.
@param {Any} [defaultValue] The default value to use if elementAt does not find a value.
@returns {Observable} An observable sequence that produces the element at the specified position in the source sequence.