/**
   * Merges the specified observable sequences into one observable sequence by using the selector function whenever all of the observable sequences have produced an element at a corresponding index.
   * @param arguments Observable sources.
   * @param {Function} resultSelector Function to invoke for each series of elements at corresponding indexes in the sources.
   * @returns {Observable} An observable sequence containing the result of combining elements of the sources using the specified result selector function.
   */
  Observable.zip = function () {
    var args = slice.call(arguments, 0), first = args.shift();
    return first.zip.apply(first, args);
  };