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

;;

foldr24 f a l = case l of Nil -> lid a; Cons x xs -> f x (foldr24 f a xs) end;

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

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

concat24 = foldr24 append Nil;