;
;(list-flatten '(9 9 (9 9 9 )))) = (9 9 9 9 9)
(define cl:list-flatten
(lambda (l)
(cond ((null? l)
'())
((atom? l)
(list l))
(#t (append (cl:list-flatten (car l)) (cl:list-flatten (cdr l)))))))