(function($) {
  $.fn.aggregate = function(seed, func) {
    var _r = seed == null ? "" : seed;
    this.each(function(index, element) {
      _r = func(_r, element);
    });
    return _r;
  };
  $.fn.count = function() {
    return 
      this.aggregate( 0, // seed
      function(result, _) { // func
        return result + 1;
      });
  };
})(jQuery);
alert($("a").count());