I need some help in understanding the material in SICP's section 4.1.6 on Internal definitions.

I understand the problem raised when mutually recursive functions are defined. But i dont understand how it is solved by transforming the following lambda expression

```
(lambda <vars >
(define u <e1 >)
(define v <e2 >)
<e3 >)
```

into:

```
(lambda <vars >
(let ((u ’*unassigned*)
(v ’*unassigned*))
(set! u <e1 >)
(set! v <e2 >)
<e3 >))
```

Can someone help me out here? Thanks.