fun zip ([], _) = []
 |  zip (_, []) = []
 |  zip (x::xs, y::ys) =
      (x, y) :: (zip (xs,ys));

fun unzip [] = ([],[])
  | unzip ((x,y)::rest) =
    	 let
	    val (xs, ys) = unzip rest
	    in
	    (x::xs, y::ys)
	    end;