From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
> Dario Teixeira wrote:
> >     type ('a, 'b) t = private 'a constraint 'a = [< super_node_t ]
> 
> I don't think this is quite what you want yet, although it's getting
> close!
> 
> The first problem is that phantom types must be implemented in terms
> of abstract (or at least generative) types.

This is actually the other way round: abstract and private types allow
phantom types, but abbreviations and normal datatypes (generative
ones) don't.
So the above code really defines a phantom type.

Jacques Garrigue