Brent, my use case is not particularly complicated. I am trying to model the pdf spec - which says that pdf contains Objects that could of of types Number, String, Name, Array and Dictionary - while array is list of objects, the Disctionary is a list of tuples (Name, Object) not (Object, Object) - hence my situation.
The WASH web framework used a trick to type HTML. If I recall correctly, they had a couple of phantom type parameters and then wrote MPTC instances for only the allowed combinations. So you'd have "HTML InXContext OrderedList" and "HTML InXContext UnorderedList", etc. Then the constructor for an X would have a type signature that required 'HTML InXContext a' of its argument. Or something like that. See if you can dig up any old WASH docs for more details.