(defun a-to-e (list)
  (cond
    ((null (cdr list)) nil)
    (t (cons
        (list (car list) (cadr list))
        (a-to-e (cons (car list) (cddr list)))))))

(defun adj-list-to-edge-list (list)
  (cond
    ((null list) nil)
    (t (append (a-to-e (car list)) (adj-list-to-edge-list (cdr list))))))