(LETREC FLATTEN
	(FLATTEN LAMBDA (L)
		(IF (EQ L (QUOTE NIL)) (QUOTE NIL)
			(IF (ATOM L) (CONS L (QUOTE NIL))
				(IF (ATOM (CAR L)) (CONS (CAR L) (FLATTEN (CDR L)))
					(APPEND (FLATTEN (CAR L)) (FLATTEN (CDR L)))))))
	(APPEND LAMBDA (X Y)
		(IF (EQ X (QUOTE NIL)) Y
			(CONS (CAR X) (APPEND (CDR X) Y)))))&