strange error message in macro

[Damien Mattei <damien.mattei@gmail.com>]

hello,
'begin' complains of 'sequence of zero expressions in form (begin)'
in:
scheme@(guile-user)> ($  {x <- 7} {y <- 8} (+ x y))
While compiling expression:
Syntax error:
unknown file:2:0: sequence of zero expressions in form (begin)
but:
scheme@(guile-user)> (begin )
make no error !
basically $ is a sort of begin macro that call :
(define-syntax %parse-assignment
  (syntax-rules ()
...

if i wrote:
(define-syntax %parse-assignment
  (syntax-rules (<-)
there is no more error:
scheme@(guile-user)> ($  {x <- 7} {y <- 8} (+ x y))
;;; <stdin>:5:0: warning: possibly unbound variable `x'
;;; <stdin>:5:0: warning: possibly unbound variable `y'
15
result is 15 and good!

i think that the explanation is with syntax-rules parameters not with
begin that would not accept zero expression in form , so why this strange
message?
i suppose i have trouble understanding the behavior and use of syntax-rules
parameters anyway and can not find any reliable documentation about it.

Damien