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

;;
id x = x;

f l n = (case l of True -> id; False -> id end) n;

{
alt l
  = case l of
       Nil -> Cons;
       Cons a as -> case as of
                       Nil -> Cons;
                       Cons b bs -> Cons
                    end 
    end;
}