This has certainly been taken into account when comparing approaches to generic programming. I quote from page 18/19 from the work you and Bulat
Indeed I was not aware of it. Missed that. Thanks for pointing it out!
Thus, full reflexivity of an approach is taken into account. This suggests constrained types are part of Haskell98. So, I'm a bit confused at the moment as well.
After reading the Haskell 98 report more carefully I think constrained types are part of Haskell98. The syntax for algebraic datatype declarations given is:
data cx => T u1 ... uk = K1 t11 ... t1k1 | ...| Kn tn1 ... tnkn
Certainly, they are implemented in a peculiar way, with constraints associated with value constructors and not the type, perhaps to keep the class and kinds orthogonal (eg, the BinTree type has * -> * kind instead of Ord -> * kind). At any rate, this has been discussed before in other threads. Thanks Thomas for your help P.