In the book **Oreilly.JavaScript.The.Good.Parts**, there is a piece of code like this:

```
Function.method('curry', function () { 
    var slice = Array.prototype.slice, 
        args = slice.apply(arguments),
        that = this; 
    return function () {
         return that.apply(null, args.concat(slice.apply(arguments)));
     };
});

```

I wonder why not directly Array.prototype.slice instead of private variables, thanks.