function concat$1(eachfn, arr, fn, callback) {
    var result = [];
    eachfn(arr, function (x, index, cb) {
        fn(x, function (err, y) {
            result = result.concat(y || []);
            cb(err);
        });
    }, function (err) {
        callback(err, result);
    });
}
Creates a function which is a composition of the passed asynchronous
functions. Each function consumes the return value of the function that
follows. Composing functions `f()`, `g()`, and `h()` would produce the result
of `f(g(h()))`, only this version uses callbacks to obtain the return values.

Each function is executed with the `this` binding of the composed function.

@name compose
@static
@memberOf module:ControlFlow
@method
@category Control Flow
@param {...Function} functions - the asynchronous functions to compose
@returns {Function} an asynchronous function that is the composed
asynchronous `functions`
@example

function add1(n, callback) {
    setTimeout(function () {
        callback(null, n + 1);
    }, 10);
}

function mul3(n, callback) {
    setTimeout(function () {
        callback(null, n * 3);
    }, 10);
}

var add1mul3 = async.compose(mul3, add1);
add1mul3(4, function (err, result) {
    // result now equals 15
});