cats.js

var execute = require("execute")
    , serial = require("serialize")

function add(x, y, cb) {
    setTimeout(function () {
        cb(null, x + y)
    }, 10)
}

var add4 = serial([
    function (a0, a1, a2, a3, cb) {
        execute({
            sum1: function (cb) { add(a0, a1, cb) }
            , sum2: function (cb) { add(a2, a3, cb) }
        }, cb)
    }
    , function (result, cb) {
        add(result.sum1, result.sum2, cb)
    }
])

var run = serial([
    function (cb) {
        execute({
            sum1: function(cb) {
                add4(1, 2, 3, 4, cb) 
            }
            , sum2: function (cb) {
                add4(5, 6, 7, 8, cb) 
            }
        }, cb)
    }
    , function (result, cb) {
        add(result.sum1, result.sum2, cb)
    }
])

run(printResult)

function printResult(err, result) {
    if (err) {
        throw err
    }

    console.log("result", result) // prints "result=10" 
}