S-99-26 Generate the combinations of K distinct objects chosen from the N elements of a list

Example:

(combination 3 '(a b c d)) ; => ((b c d) (a c d) (a b d) (a b c))

Solution:

```
(define (combination k xs)
(cond [(null? xs) '()]
[(= k 1) (map list xs)]
[else (append (map (lambda (x) (cons (car xs) x))
(combination1 (- k 1) (cdr xs)))
(combination1 k (cdr xs)))]))
```