  /**
   * 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);
  };
