    /**
     * Returns the element at a specified index in a sequence.
     * @example
     * var res = source.elementAt(5);
     * @param {Number} index The zero-based index of the element to retrieve.
     * @returns {Observable} An observable sequence that produces the element at the specified position in the source sequence.
     */
    observableProto.elementAt =  function (index) {
        return elementAtOrDefault(this, index, false);
    };
