[plt-scheme] to define, or to let

Anton, you're asking to allow programmers to specify a (currently) 
uncheckable intent. So, clearly we need more research on how to make 
such intent checkable in order to detect buggy programs. Good, get 
cracking. :)

But, until you are done and we can detect such errors we must take 
programmers at their "word" which, in my opinion, is a bad idea. In an 
implementation that is aggressive about re-ordering such things, it 
becomes very hard to ensure reliability since the behavior of the 
program may change in weird ways based on the environment in which it 
runs. To my mind, the safer point in the design space is a fixed order 
of evaluation. At least your program's behavior will be predictable so 
you can write test suites, etc.

Robby