How to use tracker autorun to queue up events

gistfile1.js

var a = {}
var b = new ReactiveVar(1);
var c = new ReactiveVar(2);

var needsUpdate = new Tracker.Dependency()

Tracker.autorun(function(){
  a.b = b.get()
  needsUpdate.changed()
})
Tracker.autorun(function(){
  a.c = c.get()
  needsUpdate.changed()
})

Tracker.autorun(function(c){
  needsUpdate.depend()
  Tracker.afterFlush(function() {
    console.log("update", a)
  })
})


// try just one at a time
b.set(3)



// then try this one
c.set(4)




// now try both of these
b.set(5)
c.set(6)