Cream Lisp: Find Common Ancestor

(defnodelistfun ancestor working-nodes ()
  "Find the common ancestor of all elements."
  (loop with parentlists = (loop for node in working-nodes
                              collect (reverse (nodefun-parents node)))
     for i = 0 then (1+ i)
     for prevparents = NIL then parents
     for parents = (loop for list in parentlists for el = (nth i list) if el collect el)
     until (or (not (every #'eql (list (first parents)) parents))
               (not (= (length parents) (length parentlists))))
     finally (return (list (first prevparents)))))