(def l '(1 2 3))
(def b 4)

(assert (= `(0 ~@l ~b) '(0 1 2 3 4)))

(defmac when [predicate & body]
  `(cond ~predicate
      (begin
        ~@body) '()))

(assert (null? (when false 'c)))
(assert (= 'a (when true 'c 'b 'a)))

(assert (=
         '(cond false (begin (quote c)) (quote ()))
         (macexpand (when false 'c))))
(assert (=
         '(cond true (begin (quote c) (quote b) (quote a)) (quote ()))
         (macexpand (when true 'c 'b 'a))))