Let's define natural numbers as following:

```
let zero: [i32;0] = [];
let one: [[i32;0];0] = [];
let two: [[[i32;0];0];0] = [];
let three: [[[[i32;0];0];0];0] = [];
```

Challenge: write addition function (or closure) capable of returning sum of two numbers.

```
assert_eq!(add(two,one), three);
```

(it should work for any previously defined number!)