list a ::= Nil | Cons a (list a); ;;

d4 xl yl
  = case xl of
       Cons x xs -> case yl of 
                      Cons y ys -> x * y + d4 xs ys;
                      Nil -> 0
                    end;
       Nil -> 0
    end;


d6 xl yl
  = case xl of
       Cons x xs -> case yl of 
                      Cons y ys -> d4 x y + d6 xs ys;
                      Nil -> 0
                    end;
       Nil -> 0
    end;

d8 xl yl
  = case xl of
       Cons x xs -> case yl of 
                      Cons y ys -> d6 x y + d8 xs ys;
                      Nil -> 0
                    end;
       Nil -> 0
    end;

d10 xl yl
  = case xl of
       Cons x xs -> case yl of 
                      Cons y ys -> d8 x y + d10 xs ys;
                      Nil -> 0
                    end;
       Nil -> 0
    end;


d12 xl yl
  = case xl of
       Cons x xs -> case yl of 
                      Cons y ys -> d10 x y + d12 xs ys;
                      Nil -> 0
                    end;
       Nil -> 0
    end;