> >    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))))))

>         I used SUBS to find all the ten-element subsets of a twenty-element
> set, in the same environment (Chez Scheme 6.1, Linux, 700MHz Pentium III)
> where SUBSETS took 460 ms and COMBOS took 250 ms for the same task.  SUBS
> took 990 ms.