Get all subsets of a set

subsets.clj

(defn combine [xs ys res]
    (if (empty? ys) res (concat (combine (cons (first ys) xs) (rest ys) '()) 
                                (combine xs (rest ys) (cons (cons (first ys) xs) res)) )))

(defn getsubsets [xs]
    (cons '() (combine '() xs '())))

(count (getsubsets '(1 2 3)))          ;; 2^3 = 8
(count (getsubsets '(1 2 3 4)))        ;; 2^4 = 16
(count (getsubsets '(1 2 3 4 5)))      ;; 2^5 = 32
(count (getsubsets '(1 2 3 4 5 6)))    ;; 2^6 = 64
(count (getsubsets '(1 2 3 4 5 6 7)))  ;; 2^7 = 128