(DE INTERSECTION (X Y)
,  (COND
,  ,  ((NULL X) NIL)
,  ,  (
,  ,  ,  (MEMBER (CAR X) Y)
,  ,  ,  (CONS (CAR X) (INTERSECTION (CDR X) Y))
,  ,  ,,,)
,  ,  ( T (INTERSECTION (CDR X) Y))
,  ,,,)
,,,)