gistfile1.el

(defun my-span (p lst)
  "Break the input list into two components -- the front part of
the list whose elements satisfy the predicate p, and the rest of
the list (at least the car of this list does not satisfy p)."
  (let (span-front span-back)
	(while (and lst (funcall p (car lst)))
	  (setq span-front (cons (car lst) span-front))
	  (setq span-back (cdr lst))
	  (setq lst (cdr lst))
	(setq span-front (nreverse span-front)))
	(cons span-front span-back)))

(my-span (lambda (x) (< x 5)) '(1 2 3 4 5 6 7 8 9 0))