x.js

// previous

var thunk1 = Thunk('one');
var thunk2 = Thunk('two');

var prevTree = h('div', [
    thunk1,
    thunk2
])

var thunk3 = Thunk('one')

var nextTree = h('div', [
    thunk3, // update called with prev === thunk1
    thunk3 // update called with prev === thunk2
])