The OCaml solution with polymorphic variants: http://www.math.nagoya-u.ac.jp/~garrigue/papers/fose2000.html http://www.math.nagoya-u.ac.jp/~garrigue/papers/fose

The OCaml solution with polymorphic variants: http://www.math.nagoya-u.ac.jp/~garrigue/papers/fose2000.html http://www.math.nagoya-u.ac.jp/~garrigue/papers/fose2000.htm.... A very good summary of that paper is available here: http://lambda-the-ultimate.org/node/1518#comment-17566 http://lambda-the-ultimate.org/node/1518#comment-17566 Another alternative based on recursive modules: http://www.math.nagoya-u.ac.jp/~garrigue/papers/#privaterows http://www.math.nagoya-u.ac.jp/~garrigue/papers/#privaterows