Benjamin Pierce's pict is the main one I'm aware of, but I think it was more an exercise than anything else (in practice if not intention): http://www.cis.upenn

Benjamin Pierce's pict is the main one I'm aware of, but I think it was more an exercise than anything else (in practice if not intention): http://www.cis.upenn.edu/~bcpierce/papers/pict/Html/Pict.html http://www.cis.upenn.edu/~bcpierce/papers/pict/Html/Pict.htm...