(def! map-nil (fn* [v l]
    "Map nil values of l to the specified value"
    (map-if v nil l)))

(def! concat (fn* [x y] (def! concat-r (fn* [x y t]
    "Concatenate arguments, keeping their order"
    (if (car x)
        (concat-r (cdr x) y (cons (car x) t))
        (if (car y)
            (concat-r '() (cdr y) (cons (car y) t))
            t))))
    (concat-r (reverse y) (reverse x) '())))

(def! distribute (fn* [x] (def! distribute-r (fn* [p n t]
    (if (empty? n)
        t
        (let* [c (car n) n (cdr n)]
            (distribute-r (cons c p) n (cons (cons c (concat p n)) t))))))
    (distribute-r '() x '())))