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

;;

lid l = case l of Nil -> l; Cons x xs -> l end;

foldr44 f a l
   = case l of Nil -> a;
               Cons x xs -> f (lid x) (lid (foldr44 f a xs)) end;

append l1 l2 = case l1 of Cons x xs -> Cons x (append xs l2); Nil -> l2 end;

concat44 = foldr44 append Nil;