Hi, I wrote a function doing the following:
it takes a list and an integer as arguments and returns a list of lists
which are all different subsets of length n from the given list:

(subset '(1 2 3 4) 2)
--> ((1 2) (1 3) (1 4) (2 3) ( 2 4) (3 4))

I would like to have your comments: can I improve the function
(that I use very much; all advice to have quicker or anything better
will be fine).
[The sublists are not given in a "pretty" order, because I don't need it;
since I take them are subsets (and not lists), (4 2) is for me the
same as (2 4)]

(define (subsets l n)
  (let ((subsets* '()))
    (letrec ((subsets0 (lambda (l2 l* n2)
                (if (zero? n2) (set! subsets* (cons l* subsets*))
                  (do ((i 0 (+ i 1)))
                      ((> i (- (length l2) n2)) subsets*)
                    (subsets0 (list-tail l2 (+ i 1))
                            (cons (list-ref l2 i) l*) (- n2 1)))))))
      (subsets0 l '() n))))