[plt-scheme] Comments on an alternate syntax for let?

> (let ()
>   (define x 1)
>   (define y 6)
>   (let ()
>     (define x (add1 x))
>     (define y (sub1 y))
>     (+ x y)))
> add1: expects argument of type <number>; given #<undefined>
I see. Internal defines behave like they are the binding parts of 
letrec, top level defines like they are the binding parts of let*.

(define x 1) 
(define x (add1 x))
(define y (add1 x))
(+ x y)

is equivalent to

(let* ((x 1)
       (x (add1 x))
       (y (add1 x)))
      (+ x y))

Am I right?

It complicates writing macros that use define, because they should work 
on both levels. Is it possible to separate the functionalities lumped 
together in define and various lets into something like

(bind x)
(assign x ...)
(scope ...)
(sequentially ...)
(simultaneously ... )?

We already have assign = set!, scope = let(), sequentially = begin. Is 
there something in PLT that can be used for "simultaneously" and "bind"?