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

filter p l
= case l of Nil -> Nil;
            Cons x xs -> case p x of
                            True -> Cons x (filter p xs);
                            False -> filter p xs
                         end
  end;

empty l = case l of Nil -> True; Cons x xs -> False end;

llist = Cons (Cons 1 Nil) Nil;

res = filter empty llist;