[Tim]
>> I'll channel that Guido would be happiest if this rule were followed:
>>
>> Given an assignment statement using "=", the meaning is the same if
>> "=" is replaced with ":=".

I'd like Guido to chime in again, because I'm pretty sure he won't
accept what's currently on the table.  There are two plausible ways to
repair that:

1. Continue down the road of making assignment expressions "exactly
like" assignment statements in their full generality.

2. Back off and limit assignment expressions to what appears to be the
overwhelmingly most common case motivated by looking at real code (as
opposed to constructing examples to illustrate pitfalls &
obscurities):

    identifier ":=" expression