Anyway, my implementation, "subs" uses up a lot of heap space compared to,
say, "combos" posted by John Stone, and incurs more garbage collection, but
still comes out faster:

(define (expand xs sets)
    (if (or (null? xs) (null? sets))
        ()
        (cons
            (apply append (map (lambda (set) (map (lambda (s) (cons (car xs)
s)) set)) (cdr sets)))
            (expand (cdr xs) (cdr sets)))
))

(define (subs xs n)
    (cond ((eq? 0 n) ())
        (else
            (do ((i 1 (+ i 1))
                 (acc (map (lambda (x) (list (list x))) xs) (expand xs acc)))
                ((= i n) (apply append acc))))))

   Sorry if I spread a bunch of misinformation with that tree-recursion
comparison: I'm still not sure why I'm wrong, but I had the same problem with
the Monty Hall thing.