function(event, newVal, oldVal)

handler(event, newValue, oldValue)

Handlers registered on "change" events will be called back as follows:

var age = compute(33);

age.on('change', function(ev, newVal, oldVal){
    ev //-> {type: "change", batchNum: 5}
    newVal //-> 34
    oldVal //-> 33
});

age(34);