don't forget to read the expression from left to right:
> currTotal := prevTotal + (prevTotal := currTotal).
5 1 4 3 2
1. the current value (X) of prevTotal is fetched.
2. the current value (Y) of currTotal is fetched
3. prevTotal is assigned currTotal which is also the value of the
4. X is sent the message + with the argument Y
5. currTotal is assigned the result from 4.