pair a b ::= Pair a b;

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

;;

pairid p = case p of Pair a b -> Pair a b end;

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

magic p = case p of Pair as bs -> Pair (lid as) (lid bs) end;

baad p = pairid (magic p);