/// A function can take a function object as an argument.
/// `twice` takes a function object; it returns a new function
/// that calls that function twice.
function plus1(x) {
    return x+1;
}function twice(fn) {
    return function(x) {
        return fn(fn(x));
    }
}

var plus2 = twice(plus1);
log(plus2(10));