code: function loop(i, callback) {
      sum(3, 4, function(){
          if (i < max) {
              loop(i + 1, callback);
          }
          else {
              callback();
          }
      })
  }
docs: /* REAL
  var max = 100;
  
  var sum = function(a, b, callback) {
  setTimeout(function(){
  callback(null, a + b);
  }, 3);
  }
  
  var sumAsync = function (a, b, callback) {
  var f = Fiber.current;
  setTimeout(function(){
  f.run(a + b);
  }, 3);
  Fiber.yield();
  }.async();