;;; library required
(reverse '(a b c d e f))
(filter '(1 2 a b 3 c 4 5 d e f) number?)
(define l '(1 2 3 4 5))
(append l '(6 7 8))
(append l '(6 7 8) '(9 10 11))
(append l '(6 7 8 (9 10 11)))
(append l 6)