why is this invalid b = a += b ?

[Stef Mientki]

hello,

I can assign a value to more than 1 variable (name) in one line:

a = b = 3

So evaluation of this line must start at the right part.

But the following is not allowed:

b = 2
a = b += 1

I would think that if b has a value,
and the formula is evaluated from right to left,
there's nothing wrong with the above formula.

thanks
Stef Mientki