When tree is deep nested, on lisp's flatten gets stack over flow.

flatten.lisp

(defun flatten(tree)
  (let((result nil)
       (head(car tree))
       (tail(cdr tree)))
    (tagbody top
      (if(and(null tail)(null head))
        (return-from flatten (nreverse result))
        (if(consp head)
	        (progn
	          (push(cdr head)tail)
	          (setf head(car head))
	          (go top))
	        (progn
	          (unless(null head)
	            (push head result))
	          (setf head(pop tail))
	          (go top)))))))