why is this invalid b = a += b ?

[Terry Reedy]

Stef Mientki wrote:
> 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

This strikes me as slightly incoherent.  Given that v op=exp is mostly 
that same as v = v op exp, I suppose you expect a = b += 1 to be mostly 
the same as a = b = b+1, but what would you do with a += b = 1, or even 
a *= b += 1.  There is some virtur to restricting augmented assignment 
to one target and one delimiter.  Even that seems to cause some trouble.

tjr